Previously published work from another laboratory has shown that the mutation pacC-5 in the ascomycete Aspergillus nidulans leads to loss of an acid phosphatase (EC 220.127.116.11) activity and is probably located in the structural gene for this enzyme. Here, we show that, pleiotropically, pacC-5 considerably reduces gamma-amino-n-butyrate transport levels as shown both by direct uptake measurements and two kinds of growth tests. A reduction in expression of the permease specified by the gabA gene is almost certainly responsible for the gamma-amino-n-butyrate uptake defect in pacC-5 strains. pacC-5 does not reduce L-proline uptake, mainly mediated by the prnB permease, or beta-alanine uptake. This work and our previously published results suggest that, although it does not uniquely reduce gamma-amino-n-butyrate uptake, pacC-5 is highly selective in its effects on transport processes. It is therefore probable that the acid phosphatase specified by the pacC gene plays some rôle in the synthesis, membrane integration or functioning of a particular class of permeases. A rôle for acid phosphatases in membrane processes casts an intriguing new light on the fact that these enzymes are periplasmic and extracellular in many micro-organisms including A. nidulans.