Fungi are the predominant cellulase-producing micro-organisms. In this study, wild-type thermophilic fungal isolates, producing highly effective cellulolytic enzymes were screened and selected using 1% carboxymethyl cellulose and combination of sweet sorghum bagasse and wheat bran under submerged and solid-state fermentations, respectively. Enzyme production among the selected 6 fungal isolates was detected within five days of cultivation under solid state fermentation Among the isolates, DIA-4 strain showed highest cellulolytic activity in 72 hours at 450C followed by potential strains IA-56 and U3. Introduction: Cellulases play a significant role in saccharifying cellulosic substrates for bioethanol production (Dhillon et al., 2011). However, production cost of these enzymes is high and accounts for 40–60% of the production cost. Hence, current research efforts are focused towards lowering the cost of enzymes. The utilization of abundant renewable lignocellulosic biomass, especially agro-industrial wastes and their by-products as substrates can help to reduce cellulase prices (Rodriguez-Couto and Sanroman, 2005). Also the use of cheaper technologies like solid-state tray fermentation can further improve the production economics (Dhillon et al., 2010). Cellulases are produced by fungi, bacteria and actinomycetes, but due to higher yields, fungi have been commercially exploited for production of these enzymes. However, majority of commercial enzymes are obtained from mesophilic fungi. Since industrial processes employ high temperatures, thermostable enzymes are in demand (Moretti et al., 2012). Further, thermophilic fungi are known to produce thermostable enzymes with activity at high temperatures, broad tolerance to pH variation and resistance to denaturing agents (Maheshwari et al., 2000; Leite et al., 2008). Keeping in view the industrial importance of the thermostable cellulases, this study was designed to isolate and select thermophilic filamentous fungi for extracellular cellulase production under submerged and solid state fermentation. Materials and methods: Source of thermophilic fungal strains Six thermophilic fungal isolates (U3, IA-56, DIA-4, KJH, L939 and L8-38) procured from School of Energy studies for Agriculture, PAU, Ludhiana, one fungal culture (Trichoderma harzianum MTCC 8230) procured from Department of Microbiology, PAU, Ludhiana and one fungal culture (Humicola insolens MTCC 1433) purchased from MTCC lab, Institute of Microbial Technology, Chandigarh were used in the present study. Potato Dextrose agar (PDA) was used for the growth (45 C) and maintenance (4 C) of fungal strains. The morphological characteristics of fungal strains including colour of mycelia, spore and growth pattern at different temperatures as well as their vegetative and reproductive structures were observed under low power light microscope using lactophenol cotton blue staining (Murray et al., 1999) Qualitative screening of cellulase producing thermophilic fungi Cellulase producing fungi were screened on selective carboxymethyl cellulose (CMC) agar containing 2.0 g NaNO3, 1.0 g KH2 PO4, 0.5 g MgSO4.7H2O, 0.5 g KCl, 10.0 g carboxymethyl cellulose sodium salt 10.0 g, 0.2 g peptone and 17.0 g agar in 1000 ml distilled water (pH 5.5-6.0). Plates were spot inoculated with spore suspension of pure cultures and incubated at 45oC. After 3 days, plates were flooded with 1% Congo red solution for 15 minutes and then de-stained with 1M NaCl solution for 15 minutes and. The diameter of zone of decolorization around each colony was measured. Cellulolytic index (CI) was determined and expressed by the ratio between the diameter of the degradation halo and the diameter of the colony (Khokhar et al., 2012). Isolates showing CI of 2.0 or more were quantitatively characterized by filter paper (FPase) assay. Quantitative determination of cellulolytic activity Submerged fermentation (SF) The selected fungal cultures were cultivated on PDA plates and incubated at 45 °C for 72-96 h after which the spores were harvested using sterile water containing Tween 80. The spore count of 1×108 spores ml-1 was used for enzyme production. Submerged fermentation using Mandel Webber medium (KH2PO4 2.0g, MgSO4.7H2O 0.3g, CaCl2.2H2O 0.3g, (NH4)2SO4 1.4g, FeSO4.7 H2O 5.0 mg, MnSO4. H2O 1.6 mg, ZnSO4.7 H2O 1.4 mg, CoCl2.6 H2O 2.0 mg, peptone1.0g, Tween 80 1.0g, pH 5.0) supplemented with 1% CMC was employed for cellulase production in 250-ml Erlenmeyer flasks, with each flask containing 100 ml of fermentation medium. The flasks were inoculated with 1 ml fungal spore suspension and incubated at three different temperatures viz. 45 °C, 50 °C and 55 °C for 5 days. A set of three flasks was removed at 24-h intervals from the incubator centrifuged and analyzed for cellulase production with respect to Filter Paper activity (Wood and Bhatt, 1988). One unit of enzyme activity was defined as the amount of enzyme required to liberate one μmole of the glucose per ml per min under standard assay conditions and expressed as international units per milliliter (IU/ml). Solid state fermentation (SSF) The selected fungal cultures were cultivated, harvested and their spores were used for enzyme production as in submerged fermentation. The Solid state fermentation was carried out in 250-ml Erlenmeyer flasks, with each flask con