Deformation movement (groove-forming activity of the equatorial surface) was induced in the spherical Tubifex egg by treatment with ionophore A23187 (5-10 μg/ml). The degree of induced movement was maximal when the treatment was commenced at the beginning of the round stages, i.e., immediately after disappearance of the grooves formed in the preceding normal deformation movement; it gradually decreased as the onset of the treatment was delayed. This induction was independent of either external Ca2+ or Mg2 +. Ultrastructural examination of eggs undergoing the induced movement revealed the presence of organized microfilaments in the cortical layer of the grooving regions. It is assumed that the absence of contractile activity during the round stages of the Tubifex egg is attributed basically to the low availability of intracellularly stored Ca2+ . The results also suggest that the distribution and/or the amount of the contractile elements change during these stages.