Hoplonemertines are carnivorous marine worms, which prey upon crustaceans and annelids. They paralyze their prey by injecting alkaloids with a stylet-bearing proboscis. The dermis of these animals also secretes alkaloids to repel predators. Besides affecting central and peripheral nervous system nicotinic receptors, some pyridyl alkaloids also activate certain chemoreceptor neurons in crustacean walking legs, which sense environmental chemicals. Anabaseine (2-[3-pyridyl]-3,4,5,6-tetrahydropyridyl) and 2,3'-bipyridyl (2,3'-BP) are two nemertine alkaloids, which potently paralyze crustaceans. Anabaseine is an agonist of vertebrate as well as invertebrate nicotinic receptors. While 2,3'-BP is non-toxic to mice, it is toxic to crustaceans. We tested a variety of nemertine pyridyl alkaloids for inhibition of barnacle (Balanus amphitrite) larval settlement and for crustacean toxicity in order to determine whether toxicity could be dissociated from inhibition of larval settlement. We prepared eight C-methylated 2,3'-BP isomers to determine where substitution is permitted without loss of activity. Anti-settlement and toxicity activities were not always related. For instance, 4'-methyl-2,3'-BP displayed only 3% of the crayfish paralytic activity of 2,3'-BP, but inhibited settlement almost 2-fold more effectively. Two other isomers displaying exceptional anti-settlement activity were the 4- and 5-methyl-2,3'-BPs; these also displayed high crustacean toxicity. Nemertelline inhibited barnacle settlement at concentrations similar to 2,3'-BP but was 136-fold less toxic when injected into crayfish. Thus, certain bipyridyls and tetrapyridyls may be useful anti-fouling additives.