The seasonal occurrence of medicinal leeches was studied at two lakes in the Black Sea coastland of north-western Turkey. The number of leeches attracted by a collector per hour was taken as an indicator for population density. Leeches were most abundant in May/June, and were almost absent during the cold season (November - March). Leeches with a weight of less than 1 g occurred principally between July and October with a peak in July. It is thought that these individuals represent newly hatched leeches. Although both study areas are close to each other, significant differences were found in respect to mean leech size and the seasonal frequency of leeches (both total number and certain age classes). It is suggested that leech biology is greatly dependent on factors such as microclimate and the availability of hosts, and no general conclusions on phenology can be drawn. There is therefore no scientific basis for restricting commercial leech collecting to certain months of the year.