Electrical recordings indicate the existence of two conduction systems, a nerve net and a slow conduction system, within colonies of the sea penPtilosarcus gurneyi.These extend throughout the colony and are separable on the basis of conduction velocity, pulse shape and threshold. Spontaneous activity in the nerve net is correlated with expansion‐contraction behavior of the colony. Activity in an expanded sea pen is less than 0.001 Hz. A burst of about 10 pulses at 0.5 to 2.0 Hz immediately precedes contraction. Nerve net pulses continue at a frequency of 0.05 to 0.07 Hz in a contracted colony. Stimulation of the nerve net at 0.5 to 2.0 Hz causes contraction, and at 0.05 to 0.07 Hz, prevents a contracted sea pen from ex‐expanding. Selective stimulation of the slow system at 0.25 to 0.5 Hz in a partially contractedPtilosarcuscauses re‐expansion, suggesting that the slow system separately exerts control over expansion. Spontaneous nerve net activity during both expansion and contraction originates primarily near the base of the rachis.