Erythromycin-resistant (Eryr) mutants of Bacillus subtilis 168 fail to sporulate at high temperature (47 degrees C) but sporulate normally at 30 to 35 degrees C. They also fail to sporulate at any temperature in the presence of 2.5 micrograms of erythromycin per ml. Neither of these nonpermissive conditions appears to affect vegetative growth, and the periods of sensitivity to both conditions extend from 40 to 90% of the sporulation period. At 47 degrees C, net incorporation of methionine and phenylalanine in postexponential Eryr and 168 cells was similar, and fractionation of the labeled products by polyacrylamide gel electrophoresis gave patterns in which many of the bands produced by mutant and parental cells coincided. However, distinct differences were seen, and since no spore-specific morphogenesis occurred in the Eryr cells at 47 degrees C, a selective defect in spore gene expression was inferred. At 35 degrees C plus erythromycin, spore morphogenesis proceeded normally until forespores were produced and then ceased, coincident with a marked increase in sensitivity of total protein synthesis to erythromycin. The effects seem to be nonspecific, therefore, and may indicate a change in cell permeability or ribosomal sensitivity to erythromycin.