Mesocyclops leuckarti increased relative to Thermocyclops oithonoides in fertilized experimental enclosures, whereas Th. oithonoides seemed to outcompete M. leuckarti in nonfertilized ones. The most reasonable factor to explain the numerical relationship between the two closely-related species was a selective pH effect : high pH favouring M. leuckarti and vice versa. In the lake the two species showed an historical development supporting the above hypothesis. M. leuckarti was the dominating species when the lake experienced highest pH during the peak eutrophic phase. Following recovery of the lake, Th. oithonoides is again becoming the more dominant species. This contrasting relationship to high pH may be the reason for the varying vertical distribution of the two species in lakes. Since Th. oithonoides is recorded below M. leuckarti, with a lower pH, temperature and food abundance, this results in the observed slower development and a more K-selected life history.