Composition and abundance patterns are described for the plankton assemblages in a shallow submarine cave ‘Grotta di Ciolo’, on the Salento Peninsula (Apulia, SE Italy) side of the Otranto Channel, between March 2002 and March 2003. Every 20 days, samples were collected at two sites inside the cave (near to and far from the entrance), and one outside, by towing 50 and 200 μm mesh sized plankton nets at each station for a horizontal distance of 30 m. Also four nocturnal collections were conducted. Temporal and spatial patterns of distribution were described, and multivariate analyses were used to assess trends in community composition. A total of 232 taxa were recognised. The cave plankton appeared more affected by the hyperbenthos than outside plankton. Mysidacea represented the most conspicuous component of the cave plankton, with two species typical in Salento marine caves, Hemimysis margalefi and Siriella jaltensis. Harpacticoida copepods, together with asexual algal propagules, dominated the plankton of the cave numerically, whereas Calanoida and Cyclopoida copepods with mollusc veligers characterised the plankton of the outside sector. While the mysid S. jaltensis was found outside the cave in nocturnal samples, H. margalefi appeared to migrate simply along the axis of the cave, never leaving it during night. The community composition changed seasonally over the 1-year period. There was clear horizontal partitioning of the plankton, with significant differences between the two stations of the cave. Higher water turbulence probably explains the presence of asexual propagules even at the inner station. An explanation of the benthos impoverishment has been proposed as an alternative to the current ‘trophic depletion’ theory.