1) The breeding succes of three-spined stickleback (Gasterosteus aculeatus L., forma leiura) males in a small stream of the Tsuya River, Gifu Prefecture, Central Japan, was studied with reference to timing of nesting initiation, use of space for nesting and social interactions. The observations were made almost daily during March to early July 1988 along the shore at a distance of 1-2 m from the fish and nests. 2) All the males in an enclosed study pool were individually marked (99 males). Furthermore, a total of 67 females were marked and observed weekly. The males were individually observed and their agonistic, courtship and parental behaviour as well as their reproductive success were quantified. Reproductive success of individual male was measured as the number of nests built, the number of successful nests, the number of hatched fry per nest. Nest sites were categorized in six types (A-F) on the basis of the proportion of vegetation cover around the nest and distance from the shore. 3) Body size and environmental factors (water temperature, water depth, changes in water level) were not correlated with reproductive success. Flooding was not a major cause of unsuccessful nesting. The brightness of nuptial colouration at the onset of breeding correlated significantly with individual success. Individual variation in the development of secondary sexual characteristics such as a nuptial colouration may have an important consequence for the lifetime reproductive success of the individuals. There was no relationship between fish density and reproductive success. 4) All males that nested more than once had begun breeding early in the season. The sooner a male started nest-building, the more opportunities he had to complete breeding cycles. 5) After an unsuccessful nest, males were significantly more likely to move their nest sites than after a successful nest. The subsequent nesting cycle was not always successful. 6) There was variation in nest-sitc location. The spatial pattern of nest distribution was strongly related to the temporal pattern, because the first males which settled, more often built their nests at sites along the shore where the nest was covered on one or two sides by vegetation. The location of nest site was significantly correlated with reproductive success. When males nested in partly concealed places along the shore, they could sometimes obtain a high reproductive success irrespective of the date of breeding initiation. Thus, reproductive success was largely determined by the timing of nest-building and nest position.