We found two patterns of parasitic mating behaviors by male in Japanese medaka, Oryzias latipes, in the spawning by one female and two males in the aquarium condition. In the first type of parasitic mating behavior, the unpaired male would rush toward the closely adhering female and paired male, and simultaneously perform emission behavior with close adhesion to the female (simultaneous emission). The second type of parasitic mating behavior was that the unpaired male would rush toward the female with spawned eggs hanging down from the genital pore several seconds after pair-spawning, perform emission behavior with close adhesion to the female without any prespawning behavior (post-spawn emission). The frequencies of “simultaneous emission” and “postspawn emission” were 20 (19.6%) and 18 cases (17.7%), respectively, in 102 trials, and the average reproductive successes about 41% and 20%, respectively. The reproductive success of simultaneous emission was not correlated with the timing and duration of spawning behaviors of the paired/ unpaired male, while the reproductive success of post-spawn emission significantly decreased as the length of time during which the paired male adhered to female increased. Observations of two consecutive spawning behaviors using same combination of two males revealed that paired males always tend to become paired males. However, the role of both males was occasionally reversed, indicating flexibility in mating tactics.