Ultrafast picosecond measurements of optically induced changes in the absolute conductivity (0.4–1.0 THz) of La0.7M0.3MnO3 thin films (M Ca, Sr) from 10 K to 0.9Tc reveal a two-component relaxation. A fast, 2 ps, conductivity decrease arises from an optically induced modification of the effective phonon temperature. The slower component, related to spin-lattice relaxation, has a lifetime that increases upon approaching Tc from below in accordance with an increasing spin specific heat. We show that, for T o Tc, ≠s≠T is primarily determined by thermally disordered phonons while spin fluctuations dominate near Tc. The observation of “colossal” negative magnetoresistance (CMR) in the hole-doped manganite perovskites (R12xDxMnO3, where, e.g., R La, Nd and D Ca, Sr) demonstrates the sensitivity of electronic conduction to the underlying magnetic structure in these materials [1,2]. Experimental and theoretical work has also revealed the importance of the lattice and orbital degrees of freedom in determining the electronic properties of CMR materials above and below Tc [3,4]. Nonetheless, it is still not clear, especially for T o Tc, what the relative importance of phonons is in comparison to double exchange in determining s. Ultrafast optical spectroscopy has provided significant insight into electron dynamics in metals [5–7], and more recently, transition metal oxides [8 –10]. Using similar ultrafast techniques, we address the relative contributions of spin fluctuations and phonons in determining the conductivity in the manganites from 10 Kt o0.9Tc. Terahertz time-domain spectroscopy is an ultrafast optical technique in which electric field transients are used to measure the complex conductivity of a material. Since this is a coherent technique, a sample can be optically excited and then probed with a terahertz (THz) pulse to measure induced conductivity changes with picosecond (ps) resolution. We use this method, known as timeresolved terahertz spectroscopy (TRTS), to measure ps conductivity transients in La0.7Ca0.3MnO3 (LCMO) and La0.7Sr0.3MnO3 (LSMO) thin films. The dynamics occur on two time scales. A fast, 2 ps, conductivity decrease arises from optically induced modification of the effective phonon temperature. The slower component, related to spin-lattice relaxation, has a lifetime that increases upon approaching Tc from below in accordance with an increasing spin specific heat. Our results demonstrate that, at low temperatures, ≠s≠T is primarily determined by thermally disordered phonons while spin fluctuations dominate close to Tc. The TRTS experiments were performed on LCMO and LSMO epitaxial thin films grown on LaAlO3 substrates using pulsed laser deposition [11]. For very thin films (150 A), island growth can alter the film properties, but the thicker films used in these experiments (1000 A) display bulk behavior [12]. Magnetization measurements