The structure-function relationships of fast skeletal myosin isoforms remain poorly understood. To shed some light, we constructed chimeric myosins comprised of Dictyostelium myosin heavy chain backbone with carp loop sequences and analyzed their functional properties. A loop 2-10 chimeric myosin having the loop 2 sequence of the fast skeletal isoform predominantly expressed in carp acclimated to 10 degrees C showed V-max in actin-activated Mg2+-ATPase activity 1.4-fold higher than a loop 2-30 chimera constructed from the loop 2 sequence of the dominant isoform in carp acclimated to 30 degrees C. These two chimera exhibited no significant differences in sliding velocity of actin filaments in in vitro motility assay. Contrastingly, both loop 1-associated chimeras, loop 1-10 and loop 1-30, did not differ in both ATPase activity and in sliding velocity of actin filaments.