The locomotor rhythmicity of the Norway lobster Nephrops norvegicus was studied under constant conditions of darkness in individuals collected on the continental slope (400–430 m). Periodogram analysis revealed the occurrence of both circadian (of around 24 h) and ultradian (of around 12 and 18 h) periodicities. Form estimate analysis of the circadian and ultradian time series revealed the occurrence of significant peaks of activity during the expected night phase of the cycle and day-night transitions, respectively. No ultradian locomotor activity rhythms have been reported in previous studies on continental shelf N. norvegicus, suggesting that this phenomenon may be limited to deep-water animals. A discussion is presented to account for the occurrence of the mechanism of ultradian rhythms when there is significant environmental light intensity reduction, as on the continental slope, where the species attains its maximum densities in the western Mediterranean.