We have tested the proposal that the light-sensitive conductance in Drosophila is composed of two independent components by comparing the wild-type conductance with that in mutants lacking one or the other of the putative light-sensitive channel subunits, TRP and TRPL. For a wide range of cations, ionic permeability ratios in wild type were always intermediate between those of trp and trpl mutants. Effective channel conductances derived by noise analysis in wild type were again intermediate (17 pS; c.f. 35 pS in trp and 4 pS in trpl) and also showed a complex voltage dependence, which was quantitatively explained by the summation of TRPL and TRP channels after taking their different reversal potentials into account. Although La3+ partially blocked the light response in wild-type photoreceptors, it increased the effective single channel conductance. The results indicate that the wild-type light-activated conductance is composed of two separate channels, with the properties of TRP- and TRPL-dependent channels as determined in the respective mutants.