The synthesis and localization of a-amylase were studied in Bacillus stearothermophilus AN002 at 40,45,50 and 55 "C and also during early exponential, late exponential and stationary phases of growth. The extracellular a-amylase synthesis was highest at 50 "C and at stationary growth phase. Accumulation of intracellular a-amylase was observed in cells grown at 40 "C and was distributed in all the cell fractions. At 50 and 55 "C the cell did not have a-amylase in the protoplasm or protoplasmic membrane. The synthesis and externalization of extracellular enzyme are two processes and the externalization is controlled by growth temperature and cell age. Considerable progress has been achieved in the understanding of exoenzyme synthesis (COLEMAN and ELLIOTT 1962, LAMPEN 1973, GLE 1976, PRIEST and THIRUNAVUKKA-RASU 1985) in bacteria. However, detailed knowledge of the niechanisms governing the expression of exoenzymes during the late exponential phase or repression during the exponential growth phase are unknown (PRIEST 1977, RAMALEY 1979, EVELEIGH and MONTENECOURT 1979). The current hypothesis for bacterial protein secretion is similar t o the model for initial synthesis and secretion of eukaryotic secretory protein into the intracisternal space of the endoplasmic reticulum ( PALADE 1975, ROTHMAN and LENARD 1977). Hence, the synthesis and secretion of exoenzyme is one process and no accumulation of exoenzyme is noticed within the cell for later secretion (SMITH et al. 1977, BLOBEL 1977, Tai et al. 1979). The above theory is substantiated by the fact that exoenzymes are not located in the cytoplasm (PRIEST 1977) and as such intracellular levels of a-amylase and protease are reported to be virtually negli-Sonication for 20 min in a 200 watt, 10 Kc, centrifugation at 145-157.