Recent investigations on the aspartate transcarbamylases (carbamoylphosphate:L-aspartate carbamoyltransferase, EC 2.1.3.2) of Escherichia coli and Streptococcus faecalis indicate that there is a site on each enzyme, apart from the active site, at which anions can bind. It is suggested in this paper that the location of such an anion binding site on the E. coli enzyme may be directly adjacent to the part of the active site at which carbamyl phosphate binds. This hypothesis is based on data demonstrating a lack of correlation between spectral changes and kinetic effects, and on a new interpretation of results obtained with N-(phosphonacetyl)-L-aspartate, which has previously been considered to act as a transition state analogue. Such a hypothesis could explain other puzzling observations made on the catalytic subunit of this enzyme, including the dependence of substrate inhibition by aspartate on the nature of the second substrate, and the ease of formation of a dead-end enzyme-aspartate-carbamyl-aspartate complex.