In soybean [Glycine max (L.) Merr.], manual cross-pollination to produce large quantities of hybrid seed is difficult and time consuming. An environmentally stable male-sterility system is one of the requirements to produce large quantities of hybrid seed. Among the many male-sterile, female-fertile mutants identified in soybean, the ms8ms8 mutant is one of the least studied. Observations of ms8ms8 plants (male-sterile, female-fertile) in different environments suggested that environmental conditions affect expressivity of the ms8 allele with respect to male sterility. The objective of this study was to determine the effect of variations in night and day temperatures on the expression of male sterility in the ms8ms8 genotype. Eighteen male-sterile, female-fertile ms8ms8 plants and ten fertile siblings Ms8Ms8 were grown in growth chambers with the following day/night temperatures regimes: 25/19, 27/21, 29/24, 25/20, 30/25, 32/25, and 28/25 ºC. Stability of the ms8 alleles was affected by the different temperature regimes. Increased male fertility of the ms8ms8 plants was observed in the regimes with the lowest day and night temperatures. Allelism tests with the partially male-sterile mutant, msp, will be conducted to determine if the ms8 and msp mutations are allelic or not.