Idotea wosnesenskii Brandt, a valviferan isopod, has five pairs of pleopods within a branchial chamber enclosed by modified uropods (operculae). The pumping mechanism of resting isopods was videotaped to observe ventilation, and particularly to determine whether or not the swimming pleopods (first three pairs) and the gas exchange pleopods (last two pairs) have ventilatory roles. Observations on intact animals (to observe water flow into and out of the branchial chamber) and on animals with operculae propped open or with parts of their abdominal wall removed (to study the actual pumping movements of the pleopods) revealed that all five pairs of pleopods function in ventilation. The ventilatory stroke has two phases: an opening phase in which the pleopods move medially and ventrally, opening spaces between pleopods (analogous to the swimming recovery stroke), and a closing phase, wherein the pleopods move laterally and dorsally, reducing the gaps between succeeding pleopods (analogous to the swimming power stroke). Ventilatory strokes may be continuous or they may be separated by resting phases. Both the frequency and amplitude are variable: the frequency changes greatly due to the wide variation in resting phase duration. By opening the operculae the amplitude can increase so that ventilatory strokes grade into swimming strokes. The ventilatory stroke pattern at rest is generally similar to the swimming stroke pattern but differs in its slower and nearly simultaneous dorsoventral movements and its lower frequencies. Dye and particle movements around intact animals and those with exposed pleopods show that water typically takes about three strokes to pass through the branchial chamber.