In 1969, a considerable amount of elvers of the European eel (Anguilla anguilla) began to be imported mainly from France and cultured in various districts of Japan, Since then, pseudodactylogyroses have occurred in eels, especially in the European eels, cultured in many farms. Pseudodactylogyrus bini, P. anguillae and a new species. P. microrchis, were reported as the causative agents of the pseudodactylogyroses by OGAWA and EGUSA (1976), but on the biology and ecology of these dactylogyrids little has been studied. Since 1974, the present authors have been studying the biology and ecology of P. microrchis. In this paper we dealt with the seasonal changes in abundance of this species in an experimental pond at Hiroshima University and two eel farm ponds in Tokushima Prefecture. The results obtained are as follows: (1) P. microrchis was observed on the gills of the European eels cultured in those three ponds. Besides, P. bini was observed in the experimental pond. (2) It was found that the size of the testis and the size ratio of the testis to the ovary cannot always be used as the discriminating character of P. microrchis from P. anguillae. (3) An apparent seasonal change in abundance of P. microrchis was observed in every pond, that is, the number of parasites became most abundant in August, September or October. (4) From the fact that the each parasite population attained its peak in the above mentioned season after high water temperature period (26°C-30°C), the abundance of this species is thought to closely depend on the water temperature.