The joint action of a plant parasitic nematode, Pratylenchus penetrans (root-lesion nematode), and an insect defoliator, Leptinotarsa decemlineata (Colorado potato beetle), on growth, development, and yield of Solanum tuberosum cv. Superior was studied in the field. Three population densities of P. penetrans were superimposed on each of three population levels of L. decemlineata. The major impact of P. penetrans on final yield was through a reduction in the number of tubers formed during tuber initiation. Defoliation by L. decemlineata increased with time as larvae advanced through successive instars and densities increased. This resulted in a significant reduction in tuber weight and numbers. Total yield of S. tuberosum was decreased by 66% with increasing population densities of L. decemlineata and 27 % with increasing densities of P. penetrans. L. decemlineata feeding did not affect soil population densities of P. penetrans. Root population densities of P. penetrans, however, were significantly (P = 0.05) higher in plants maintained beetle free than in plants grown in the presence of the beetles. Key words: yield, crop loss, interaction, root-lesion nematode, Colorado potato beetle, potato.