The effects of interaction across a circular aperture in a plane, perfectly conducting screen are studied by application of the approximate edge-current diffraction theory previously developed,1the interaction terms being found by asymptotic evaluation of the first-order aperture field. Their effect is examined by computing the aperture and axial fields and comparing the results with exact theory. An interaction correction to the geometrical-optics transmission coefficient, which agrees well with rigorous theory except for a monotonic component, is obtained. With an assumed current distribution over the entire screen, an expression is found for the transmission coefficient, neglecting interaction, which exhibits monotonic behaviour at variance with that expected from exact theory. A term with the desired variation is derived empirically.The possibility of extending the method to apertures of more general form is briefly mentioned.