This study reports the effect of vesicular-arbuscular mycorrhizal (VAM) fungi on dry matter production by Pachycereus pecten-aboriginum (Engelm.) Britt & Rose, an arborescent cactus of arid and tropical dry forest in Mexico. Seedlings in the presence or absence of VAM fungi were grown in soil between two plates of glass (20 × 30 cm) for 8 months inside growth chambers (30/25° C, 13/11 h day/night and a light intensity of 400 μmol m-2 s-1). VAM seedlings had significantly (P<0.01) higher dry matter production (0.418 versus 0.169 g), root/shoot ratios (0.26 versus 0.14) and specific root length (0.65 versus 1.41 mm mg-1) than non-VAM seedlings, suggesting a more efficient exploitation of soil resources by the VAM cacti. The data point to a role for VAM fungi in the establishment, growth, water relations and nutrition of cacti in the arid tropics.