The effect of sixteen metal ions: Al, Cd, Co(II), Cr(III), Cr(VI), Cu(II), Fe(III), Li, Mg, Mn(II), Mo(VI), Ni(II), Pb(II), Se(IV), V(V), and Zn on the mortality and infectivity ofHeterorhabditis bacteriophora were observed over a 96 hr period. All ions except Pb(II) even at naturally unrealistic concentrations did not cause the mortality of the nematodes. A weak vitalizing effect could eventually be observed with Mn(II), Mg, Fe(III) and Ni(II) (Table 1). However, such treatment generally lowered infectivity of the nematodes with respect to wax moth caterpillars.Galleria mellonella. This effect was particularly significant with Ni(II) and Pb(II).