A study was made on the Pacific sardine (Sardinops sagax) off the coast of western Mexico to determine the effect of temperature on spawning and early life in the egg stage in order to assess the daily egg production. Variations in development time and spawning of S. sagax were recorded over 24 hour cycles. A drift float was used for recording temperature at surface, at mid-depth, and at the bottom. Zooplankton samples were taken in Magdalena- Almejas Bay, B.C.S., during the spawning peak from December 1984 to February 1986. Temperature variations were observed during the day, together with an increase in the embryo development during part of the day. These generated differences between development times. estimated by two different models. A wide spawning range of temperature (16ºC to 27ºC) was confirmed for this species, with discontinuities observed at 17ºC and 22ºC . The highest frequency of spawning was at 22:00 hours. The reproduction characteristics of sardine as a function of temperature are discussed, as it represents one of the environmental factors controlling reproduction and spawning.