We examined the effects of heavy metals on the development of horseshoe crabs using bioassays in which embryos at different stages were exposed either continuously or for 24 h. Abnormalities included segment-defective embryos, double embryos, embryos with abnormal eye areas, and no-posterior embryos. In terms of their ability to affect normal development, the pollutants are ranked as follows: Hg >== organotin > Cr falling dots Cd > Cu > Pb > Zn. Organotin treatment was associated with a high frequency of abnormal eye embryos and Hg with segment-defective embryos. The specific cause of segment-defective malformations by Hg was investigated by using SH inhibitors and cytochalasin B, an inhibitor of microfilaments. These inhibitors induced Hg-type segment-defective embryos; thus, Hg may also act by inhibiting SH-SS exchange and blocking morphogenetic movements. Horseshoe crab embryos from Delaware Bay and Sandy Hook Bay, New Jersey, were examined for developmental abnormalities. Malformed embryos were extremely rare (<0.6%) at five beaches along Delaware Bay as well as Sandy Hook Bay, suggesting that these horseshoe crab spawning areas are relatively unpolluted by heavy metals.