The synchronous neurotransmitter release that results from repetitive firing is often accompanied by asynchronous release that occurs later and is not tightly synchronized with presynaptic action potentials. Asynchronous firing is thought to be a consequence of an expansion of the intracellular calcium domains that trigger the synchronous release. However, this study suggests that, at zebrafish neuromuscular synapses, these two types of neurotransmitter release depend on different calcium stores: synchronous release involves calcium entry via P/Q-type calcium channels, whereas asynchronous release depends on calcium waves propagating from outside the synapse.