Streptomyces albus G mutants (at least 12 of which were independent) defective in SalGI-mediated restriction (R-) were isolated after mutagenesis. Some of them lacked detectable SalGI activity in cell-free extracts. Some were also partially or completely defective in SalFI-associated modification (M-). Loss of restriction rendered S. albus G sensitive to many phages to which it was normally totally resistant. DNA from one such phage had many SalGI target sites (mean, one site per 1.35 kilobases). A mutant was isolated which was heat-sensitive for growth, apparently because it was restriction-proficient but temperature-sensitive for modification. At a rather high frequency, this mutant generated spontaneous heat-tolerant derivatives which were nearly all R-. Such R- mutants were always M- rather than being temperature-sensitive for modification. In a limited genetic analysis, the determinants of restriction and modification did not recombine with each other, and since there was no reassortment of these phenotypes among the parental output of crosses it appeared that the determinants were located close together on the chromosome.