lexAop regulatory sequences govern expression of the GCaMP6s Ca[2+] indicator, tethered to the C-terminus of the vesicular synaptic protein Syt1 with an intervening 3xGS linker, such that the GCaMP6s sensor is outside the synaptic vesicle. The GCaMP6s sensor is co-localized with endogenous synaptic proteins and can be used to detect Ca[2+] influx specifically at presynaptic terminals.