The myosin heavy chain (MHC) and myosin light chain (MLC) isoforms in skeletal muscle of Rana pipiens have been well characterized. We measured the force-velocity (F-V) properties of single intact fast-twitch fibers from R. pipiens that contained MHC types 1 or 2 (MHC1 or MHC2) or coexpressed MHC1 and MHC2 isoforms. Velocities were measured between two surface markers that spanned most of the fiber length. MHC and MLC isoform content was quantified after mechanics analysis by SDS-PAGE. Maximal shortening velocity (V_max) and velocity at half-maximal tension (V_P 50) increased with percentage of MHC1 (%MHC1). Maximal specific tension (P_o/CSA, where P_o is isometric tension and CSA is fiber cross-sectional area) and maximal mechanical power (W_max) also increased with %MHC1. MHC concentration was not significantly correlated with %MHC1, indicating that the influence of %MHC1 on P_o/CSA and W_max was due to intrinsic differences between MHC isoforms and not to concentration. The MLC3-to-MLC1 ratio was not significantly correlated with V_max, V_P 50, P_o/CSA, or W_max. These data demonstrate the powerful relationship between MHC isoforms and F-V properties of the two most common R. pipiens fiber types.