Twelve Streptomyces uenezuelae mutants blocked in chloramphenicol biosynthesis were isolated. Two of these (Cml-1 and Cml-12) were apparently blocked in the conversion of chorismic acid to p-aminophenylalanine and three (Cml-4, Cml-5 and Cml-8) accumulated p-aminophenylalanine and may have been blocked in the hydroxylation reaction that converted this intermediate to p-aminophenylserine. One mutant (Cml-2) accumulated D-threo-1 -p-nitrophenyl-2-propionamido-1,3-propanediol and D-threo-1 -p-nitrophenyl-2-isobutyramido- 1,3-propanediol, indicating that chlorination of the a-N-acyl group of chloramphenicol was blocked. The remaining six strains did not excrete any detectable chloramphenicol pathway intermediates.