Reproductive behavior of the Japanese dipper, Cinclus pallasii studied in spring, 1978 in Hakone National Park, Japan, and the American dipper, Cinclus mexicanus, studied in spring 1977 in the Sierra Nevadafoothills, California, are described and their mating systems examined. Similarities between the two species were as follows. 1) Both species were living in an area frequented by man, and in fact built their nests in man-made walls that formed dams. 2) Both species showed rigid territoriality, and rarely crossed the boundaries of their territories. Males and females engaged in territorial defense by means of the territorial defense flight. 3) The sequence of events during the reproductive season varied from what is usually seen in birds in that the pair bond was formed before courtship displays were performed. 4) The male of both species switched mates during the nest building phase. Both the old and new mate were later present together in the nest area when courtship and/or copulation took place, and in the case of C. mexicanus, the male copulated with both females. 5) Females showed partial asynchrony in reproductive cycles in adjoining territories.<br>Differences were also observed. 1) Nests were situated in adjoining territories so that they occurred closer together for C. pallasii than for C. mexicanus. 2) C. pallasii showed a greater tendancy to fly over land rather than to follow the course of the stream as C. mexicanus did. 3) The courtship display performed by C. mexicanus was what I called the vertical stretch display, while a horizontal stretch display was performed by C. pallasii. Both species performed the circular display flight.<br>I hypothesize that the striking similarities in reproductive behavior of C. mexicanus and C. pallasii are due largely to the similarities of the environment in this study, especially the effects man has on their habitat. The presence of man and his influences on the habitat through the building of dams, bridges, and fish hatcheries, works to limit or clump resources critical to the nesting behaviors of C. mexicanus and C. pallasii, especially nesting sites and feeding areas. This factor, along with the preadaptations of the two species of dippers for polygyny, through unequal parental care, the use of closed nests, allowing for less parental care, asynchrony in the reproductive cycles of females, and utilization of an aquatic food supply, allows these species to adopt a polygynous mating system. Inversely, since dippers are so well adapted to a polygynous mating system, they are able to cope with changes in their environment brought on by man.