AbstractColonies of Labyrinthula, a colonial marine protist, expand by protrusive movements of the specialized slimeways. The movements recorded in time‐lapse films are of two types – filopodial and lamellipodial – and occur at rates equivalent to those of cell translocation.Evidence is presented that Ca2+regulates the contraction of the actomyosin system of filaments present in the slimeways of Labyrinthula. In glycerinated models or in colonies exposed to ionophore A23187 contraction is evidenced by the occurrence of periodic contractions of the slimeways, giving them the appearance of strings of beads. Glycerinated slimeways contract on the addition of Ca2+and ATP while slimeways provided with ionophore A23187 contract on addition of Ca2+alone. The concentration required is 1.1 × 10−7M Ca2+while concentrations of 6.2 × 10−8or lower were ineffective. Rates of contraction were measured in time‐lapse films which provide evidence that contractions and beading occur everywhere in the slimeway system. When beading occurs, the 6‐nm filaments transform from an array of parallel single filaments into an interwoven meshwork.We have identified by pyroantimonate‐OsO4fixation, as possible Ca2+reservoirs, deposits of Ca2+in bothrosomes – structures through which cell secretions pass into the slimeways. The electron‐dense deposits are located at the base of the bothrosome and disappear after incubation with EGTA. We propose that the translocation of cells as well as the movements of slimeways may be regulated by the cells through the local measured liberation of Ca2+from the bothrosome wh