Ovarian development in Cyprinodon rubrofluviatilis was relatively unaffected by the daylength treatments (7 hr photoperiod vs 15 hr period) compared with the effects of the test temperatures (15'C vs 300C). Large eggs near maturity were produced by all experimental groups, but individuals of the 150C groups had greater mean egg diameters and gonosomatic indexes than those of the 30'C groups. The latter groups had much greater coefficients of condition (KTL) than the former, apparently as a result of higher energy intake and high metabolic efficiency. The explanation of the differential ovarian development may lie partially in a competitive relationship between ovarian and somatic mass, with high proportions of assimilated energy shunted to somatic tissues at high temperatures. The pupfish seems to have low temperature thresholds for reproductive functions (ovogenesis and spawning), and this conforms well with the apparent ecological role of this species. In this paper we describe the effects of four temperature-daylength combinations on ovogenesis during the interval between spawning seasons in the Red River pupfish, Cyprinodon rubrofluviatilis, a com- mon cyprinodontid in the Red and Brazos river systems of west Texas and Oklahoma. As Harrington (1959) noted for another cyprinodon- tid, Fundulus confluentus, under similar experimental conditions, any possible influence of daylength seemed secondary to temperature ef- fects. For the genus Cyprinodon there is a relatively large body of information on the relationship between temperature and various bio- logical functions; e.g., upper and lower temperature tolerances (Lowe and Heath, 1969; Brown and Feldmeth, 1971), developmental rate and upper limit for successful incubation (Kinne and Kinne, 1962; Echelle, Hubbs and Echelle, 1972), territoriality and spawning (Echelle, 1973), feeding rate and metabolic efficiency (Kinne, 1960),