Nothobranchius guentheri, an annual fish from East Africa, is found in temporary ponds, pools, and streams that dry seasonally. The populations survive the dry seasons in the form of thickly chorionated embryos encased in the muddy substrates. When N. guentheri adults are allowed to spawn in the laboratory at 24.0 +/- 1 degree C and the fertilized eggs are collected shortly afterwards, formation of the embryonic axis normally occurs within 5 to 11 days. However, examination of embryo populations obtained from unaerated aquaria after uninterrupted spawning periods of up to 2 or 3 months revealed only rare instances of embryogenesis. The overwhelming majority of embryos proceeded no further than the dispersed cell phase (diapause I, stage 20). Spawning was found to be continuous throughout these prolonged periods and the possibility of adult predation was eliminated. While low PO2 has been determined to be a factor contributing to the duration of diapause I, determinations of PO2, PCO2, pH, free ammonia and bacterial flora indicated that aquarium conditions could support embryogenesis. Only embryos at stages 20 and 32, both being stages of naturally occurring diapause, could be inhibited. The inhibitory effect was dependent on the adult density and was diminished with aeration. It is proposed that, in nature, the adult-produced inhibitory factor may be useful in the conservation and augmentation of the embryonic reserve which is the mainstay of annual fish populations in tropical areas of alternating rainy and dry seasons.