The effects of the spider toxin alpha-latrotoxin (alpha-LTX) on gametes, zygotes, and early embryos of the sea urchin Paracentrotus lividus have been investigated by in vivo experiments and by histochemical studies of acetylcholinesterase (AChE) activity. Treatment of unfertilized eggs with nanomolar amounts (1 to 0.3 nmol/l) of alpha-LTX neither triggered cortical granule exocytosis, nor prevented the elevation of the fertilization layer by sperms. Instead, fertilized eggs exposed to alpha-LTX showed noticeable alterations in cell surface topography, including the appearance of prominent membrane-limited blebs. Moreover, the zygotes treated with 1 nmol/l alpha-LTX failed to cleave. The histochemical staining of treated zygotes revealed a very strong AChE activity in the cortical region, including blebs. An enzyme reaction was also found in the perivitellin space. Our results suggest the hypothesis that some alpha-LTX receptors may appear after fertilization, supporting the awareness that fertilized eggs display excitable cell features.