1. The close association of muscle and neurons in Ascaris suum makes it difficult to determine whether spikes recorded from nerve cords originate in muscle or neurons. We have developed criteria that distinguish muscle and neuronal activity. There are two categories of extracellular spikes. 2. The first category consists of spikes with a wide range of amplitudes, marked by large spikes. These spikes, which can be recorded over lateral muscle and over the dorsal and ventral nerve cords, are abolished when muscle is disrupted or removed, or when curare is applied. Large spikes are relatively infrequent, are correlated with intracellularly recorded muscle events, and respond to polarizations of motor neurons, implying that they originate in muscle. 3. The second spike category, small amplitude spikes, is exclusive to the ventral nerve cord, occurs more frequently than large spikes and displays patterned firing. Small spikes are not affected by muscle removal or by curare, and are correlated with motor neuronal post-synaptic potentials, but not with intracellularly recorded muscle events. We infer that they originate in neurons. 4. Low level activity recorded extracellularly over nerve cords may represent muscle activity due to tonic motor neuronal synaptic transmission. It responds to motor neuronal polarization and is suppressed by curare or muscle removal.