Asexual organisms are thought to gain an advantage by avoiding the cost of producing males. In the cladoceran Daphnia pulex (Leydig), male production is determined by the environment and is independent of the origin of the asexual obligate parthenogens from the sexual cyclical parthenogens. If there is a cost to producing males, successful obligate parthenogens should have reduced or eliminated male production. Field and laboratory observations showed that obligate parthenogens have much-reduced male production compared to cyclical parthenogens. Although the reduction or elimination of males in the obligate parthenogens suggests that the cost of males is avoided, the coexistence of sexual and asexual forms of D. pulex may be partially explained by cyclical parthenogens compensating for the cost of males by having greater fecundity. In addition, the absence of a mating constraint for the obligate parthenogens may favour an increased allocation to asexual diapausing eggs earlier in the season compared to the cyclical parthenogens which require mating with males to produce sexual diapausing eggs. No difference in the production of diapausing eggs was observed, probably because males were abundant in populations of cyclical parthenogens and do not appear to limit the production of sexual diapausing eggs. D. pulex is a useful system for determining the ecological consequences of abandoning sexual reproduction and explaining the coexistence of sexual and asexual forms of a species.