Molar Cotton-Mouton constants at infinite dilution in carbon tetrachloride are reported; from these, in conjunction with available polarizability and susceptibility data, the diamagnetic anisotropies of the solutes are evaluated. Using known molecular dimensions the π electron ring currents appear, relatively to benzene as 1, to be 1.06 : 0 .19 : 0.57 : 0.37 respectively for the four heterocycles in the title.