In contrast with wild-type, Syx1A^Δ229 and Syx1A⁰⁶⁷³⁷ flies rescued with Syx1A^H3-C, undergo no peristaltic muscle contractions prior to hatching, and show a poor response to tactile stimulation. Syx1A^Δ229 embryos rescued with Syx1A^H3-C appear thinner and somewhat irregular compared to wild-type, though cuticle is secreted normally. Using whole-cell patch-clamp analysis, about 70% of Syx1A^H3-C mutants in a Syx1A^Δ229 null background show an evoked neurotransmission that is strongly reduced when compared to wild-type. In the remainder, the evoked response is abolished. Mutants reveal robust postsynaptic responses, similar to controls. mEJC frequency is also significantly increased as compared to controls. Mutants exhibit a decreased reliability of excitation-secretion coupling: evoked neurotransmission in these mutants is characterised by asynchronous release, low fidelity of release, a high failure rate and increased variability of response. This suggests defects in the ability of the fusion machinery to properly regulate synchronized release of synaptic vesicles in response to Ca²⁺ influx.