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A massive and apparently total mortality of white-clawed crayfish (Austropotamobius pallipes complex) occurred in Rossenna (September 2004), Lerna (February 2005) and Selve (June 2005) streams in Modena province, Northern Italy. In all three streams the deceased crayfish were in different states of preservation and only recently dead or moribund crayfish were collected and submitted to mycological, parasitological, bacteriological, virological, and histopathological analyses. Extended Biotic Index (EBI) was evaluated for each water course and chemical-physical and bacteriological water analyses were carried out., Saprophytes, ectosymbionts and ectocommensal organisms (sessile peritrich ciliate protozoa and branchiobdellida) were frequently observed in crayfish. No virus was found. Fusarium sp. was the most frequently isolated mycete and it occurred in pure culture from the specimens coming from creek Selve. Furthermore Mucor sp. and Penicillium sp. were detected on the crayfish from the river Rossenna, while Trichoderma sp., Alternaria sp., Gliocladium sp. and Mucor sp. from Lerna specimens. The bacteriological investigation recorded Hafnia alvei from both Lerna and Rio Selve samples, while Aeromonas hydrophila only from Lerna. During the histological investigation,, all crayfish showed severe fungal hyphae infiltrations from the exoskeleton through the superficial skeletal to the muscle hypodermis evidencing inflammatory reactions and melanization. All water courses showed environmental pollution. A prolonged exposure to toxic substances in the water could have exhausted the defense mechanisms so that the crayfish were more susceptible to the action of opportunistic pathogenic agents and subsequently succumbed.