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The influence of urea on actin-activated myosin Mg2+-ATPase of requiem shark Triakis scyllia was examined and compared to that of carp (Cyprinus carpio). No changes in the maximum turnover rate, V-max, was observed for shark myosin up to 0.3 M urea which is an approximate physiological concentration in marine elasmobranch tissues. The rate for carp myosin decreased by 70% at this urea concentration. The affinity of myosin to actin also remained unchanged up to about 0.7 M urea for shark in marked contrast to carp myosin which decreased by 30%, even in 0.1 M urea. Since it has been reported that urea-resistibility of requiem shark myosin Ca2+-ATPase in the absence of actin is comparable to that of carp, a high resistibility against urea in actin-activated Mg2+-ATPase of shark myosin is partly accounted for by its stable interaction with actin. (C) 1999 Elsevier Science Inc. All rights reserved.