Expression levels of Bacillus amyloliquefaciens alpha-amylase, Escherichia coli TEM-beta-lactamase, and Semliki Forest virus glycoprotein E1 genes were compared in Bacillus subtilis. All three model genes were expressed by using a secretion vector, constructed by joining the B. amyloliquefaciens alpha-amylase promoter and signal sequence with plasmid pUB110 (I. Palva, M. Sarvas, P. Lehtovaara, M. Sibakov, and L.Kääriäinen, Proc. Natl. Acad. Sci. U.S.A. 79:5582-5586, 1982). When transformed B. subtilis cells were grown to early stationary phase, the amount of beta-lactamase in the culture medium was ca. 10% and that of E1 was ca. 0.01% of the amount of alpha-amylase. The amounts of specific, full-length transcripts of the cloned genes were estimated by Northern blot hybridization to be roughly equal. The half-lives of these transcripts in B. subtilis were also similar. Pulse-chase experiments with [35S]methionine showed that alpha-amylase and beta-lactamase were translated and secreted at comparable rates but that beta-lactamase was degraded during the chase periods. In transformed minicells from B. subtilis, the products of alpha-amylase, beta-lactamase, and E1 genes accumulated at similar rates. We conclude that the expression of the three genes cloned in the secretion vector was similar at the levels of transcription and translation in B. subtilis. In the case of beta-lactamase, the low-yield could be explained by proteolytic degradation of the secreted product by B. subtilis exoproteases, whereas with E1 we could not determine whether the low yield was due to proteolytic degradation, inefficient secretion, or both.