In pinacocytes around the base of fresh-water sponges (Spongillidae) mitochondria move over long, straight paths, at an average speed of 1.3 micron/s. The application of colcemid (10(-4) M) has a distinct influence on mitochondrial movement: after 45 min the paths are dramatically shortened, after 90 min the periphery is devoid of mitochondria, and after 180 min all the mitochondria have aggregated in the perinuclear region. Three hours upon removal of colcemid, most mitochondria start to migrate away from the nuclear region along new paths and reach the boundary of the cell after 4 h of reculturing. These dynamics were analyzed in serial video frames. Immunocytochemical staining of comparable pinacocytes with a monoclonal tubulin-antibody reveals a characteristic system of unbranched, radially arranged microtubules extending from the nucleus to the boundary of the cell. After the application of colcemid (10(-4) M), progressively fewer and shorter microtubules are labeled. Four hours after removal of colcemid, the microtubular system is restored. The results of the present paper imply that mitochondrial movement in pinacocytes is dependent on the existence of an intact microtubular system.