Abstract Muscle fibers from Drosophila larvae show an L-glutamate-sensitive membrane potential. Bath-applied L-glutamate depolarizes the muscle in the range from 0.5 to 20 μM. Greater concentrations of the agonist repolarize the fibers. The repolarizing effect disappears if chloride is replaced by sulfate in the external medium. Intracellular recordings show the occurrence of depolarizing and hyperpolarizing spontaneous miniature postsynaptic potentials (smpp). Patch-clamp studies indicate the presence of two types of receptor channels: (i) an anion-selective channel activated by both L-glutamate and GABA. In outside out-patches, bathed in symmetrical 140 mM Cl − and 200 μM GABA, the channel displays conductance substates of 40, 80 and 110 pS. In the presence of 200 μM L-glutamate only the 40 and 80 pS substates are observed; (ii) a cation-selective channel activated only by L-glutamate that has a conductance of 104 pS in cell-attached patches (128 mM Na + outside). The presence of these two types of receptor channels in Drosophila muscle may explain the effect of bath-applied L-glutamate on membrane potential and the presence of inhibitory and excitatory smpp.