We investigated the Ca²⁺ channel-synaptic vesicle topography at the inhibitor of the crayfish (Procambarus Clarkii) Neuromuscular Junction (NMJ) by analyzing the effect of different modes of Ca²⁺ channel block on transmitter release. Initial identification of Ca²⁺ channels revealed the presence of two classes, P- and non-P-type, with P-type channels governing ~70% of the total Ca²⁺ influx. The remaining Ca²⁺ influx was completely blocked by Cd²⁺, but not by saturating concentrations of -conotoxins MVIIC and GVIA, or Nifedipine and SNX-482. To examine the relative spatial distribution of Ca²⁺ channels with respect to synaptic vesicles, we compared changes in IPSC amplitude and synaptic delay resulting from different spatial profiles of [Ca²⁺]_i around release sites. Specifically, addition of either [Mg²⁺]_o, which decreases single channel current, or -Aga IVA, which completely blocks P-type channels, prolonged synaptic delay by a similar amount when Ca²⁺ influx block was <40%. Because non-P-type channels are able to compensate for blocked P-type channels, it suggests that these channels overlap considerably in their distribution. However, when Ca²⁺ influx was blocked by ~50%,-Aga IVA increased delay significantly more than Mg²⁺, suggesting that P-type channels are located closer than non-P-type channels to synaptic vesicles. This distribution of Ca²⁺ channels was further supported by the observations that 1) non-P-type channels are unable to trigger release in physiological saline, and 2) EGTA preferentially prolongs synaptic delay dominated by non-P-type channels when transmitter release is evoked with broad action potentials (APs). We therefore conclude that though non-P-type channels do not directly trigger release under physiological conditions, their distribution partially overlaps with P-type channels.