Low activities of the monomeric aluminium (Al) species, Al3+, Al(OH)2+ and Al(OH)2+ in solution reduce root growth and root hair development in soybean [Glycine max (L.) Merr.]. Calcium (Ca) ameliorates to a certain extent the toxic effects of Al on root growth, but the interactive effects of Ca and Al on root hair development are not known. In the absence of added Al in solution culture, Ca concentration over the range 500–2000 μM had little effect on root growth or root hair development of soybean cv. Fitzroy. Where the sum of the activities of the monomeric Al species was 2 μM, taproot elongation and lateral root development of soybean was suppressed in solution with 500 μM Ca. The length of the root hair zone was only 10% of that in plants not exposed to Al, and scanning electron microscopy revealed a low density of root hairs. Increasing the Ca concentration to 2000 μM largely overcame these deleterious effects. The results are discussed in relation to the role of root hairs as infection sites for Bradyrhizobium, and the known effects of Al in suppressing nodulation in soybean.