Although Cu is phytotoxic at Cu 2+ activities as low as 1–2 μM, the effect of Cu 2+ on the nodulation of legumes has received little attention. The effect of Cu 2+ on nodulation of cowpea ( Vigna unguiculata (L.) Walp. cv. Caloona) was examined in a dilute solution culture system utilising a cation exchange resin to buffer solution Cu 2+. The nodulation process was more sensitive to increasing Cu 2+ activities than both shoot and root growth; whilst a Cu 2+ activity of 1.0 μM corresponded to a 10% reduction in the relative yield of the shoots and roots, a Cu 2+ activity of 0.2 μM corresponded to a 10% reduction in nodulation. This reduction in nodulation with increasing Cu 2+ activity was associated with an inhibition of root hair formation in treatments containing ≥0.77 μM Cu 2+, rather than to a reduction in the size of the Rhizobium population.