SUMMARY Arginine biosynthesis in Pseudoinonas aeruginosa proceeded via transacetylation of acetylornithine with glutamate ; it resembled Micrococcus glutamicus rather than Escherichia coli. Of four arginine biosynthetic enzymes, N-acetyl-y-glutamokinase, N-acetylornithine glutamate transacetylase, ornithine transcarbamylase and argininosuccinase determined under various conditions of arginine excess and deprivation, only ornithine transcarbamylase (OTCase) varied. It appeared to be fully derepressed in the wild-type strain grown in minimal medium and partially repressed in the presence of arginine. OTCase was also partially repressed in all arginine auxotrophs, even when deprived of arginine. OTCase was derepressed in bradytrophic revertants of arginine auxotrophs grown in minimal medium, but its level was never greater than that in the wild-type. The level of endogenous arginine in the wild-type strain grown in minimal medium is probably insufficient to effect repression. Exogenous arginine is more effective in repression when present as the sole carbon source.