SUMMARYStudy of the ultrastructure of dividing nuclei in the somatic phase of the life cycle of the myxomyceteClastoderma debaryanumhas revealed a mitotic apparatus that is acentric (without centrioles) and has a continuous nuclear envelope. Groups of spindle microtubules of the metaphase mitotic apparatus converged slightly and terminated on or near the inner membrane of the nuclear envelope to produce at the flattened“poles”microtubule configurations that were reminiscent of those seen when microtubules are associated with centrioles. It is suggested that the myxomycete mitotic apparatus may contain an evolutionary forerunner of the centriole of higher organisms.