Allometric variation and insect seed predation among Arctium minus, A. lappa, and two possible hybrids were quantified. All four biotypes were also examined using isoelectric focusing, and differences among their proteins were found for alkaline phosphatases, leucine amino peptidases, esterases, and peroxidases. The allometric and electrophoretic results indicated that hybridization between A. minus and A. lappa is very common. The implications for the population dynamics of Arctium are discussed. The parental species are probably maintained by the combined effects of differential insect predation on the seeds, differential dispersal of seeds, and reduced seed viability in hybrids.