Two forms of M. p. pruinosa males occur in the study area (Ino river, Japan), viz. f. esakii and f. strigata. Territorial behaviour between strigata and esakii males is initiated only by the latter, and usually leads to displacement of slrigala. The behaviour of slrigala males in the absence of esakii. was observed following removal of esakii males from the experimental area. M. p. p. strigata males established territories, and drove away other intruding slrigala. They also showed the same reproductive behaviour (except courtship) as esakii males when females appeared in the territory. The mean frequencies of copulations observed for slrigala males under natural and experimental conditions were 1.0 ± 1.2 (95% confidence limits) times and 1.1 ± 1.1 times per 20 min, respectively. This suggests, therefore, that the frequency of copulations for slrigala does not increase when esakii males are removed. The pre- and post- copulatory mating strategies of both esakii and strigata males are discussed.