The causative factors for corn (Zea mays L.) response to methyl bromide (CH3Br) fumigation, in the absence of known specific pathogens, are unknown. The study was conducted to determine if deleterious nonspecific rhizosphere microorganisms are the causitive agents. Soil fumigation was postulated to increase yield of continuous corn but to have less effect on the yield of corn grown in rotation, because more deleterious rhizosphere microorganisms were suspected in monoculture than in rotation. The effects of fumigation and crop rotation on corn grain yield, plant height and P content, available soil N and P, mycorrhizal infection, and soil microbial biomass were investigated at two sites near Mead, NE, on a Sharpsburg silty clay loam soil (fine, montmorillonitic, mesic Typic Argiudoll). Methyl bromide treatments were applied prior to planting for 4 yr at one site and 2 yr at the other site. The sites differed in fertilization, pest management, and crop sequences, but both contained continuous corn. Fumigation resulted in an increase in grain yield under monoculture only once in six site years. Unexpectedly, however, fumigation resulted in a decrease in grain yield under rotation in 7 or 12 observations. Plants in fumigated soil were P-deficient early in the growing season despite similar soil test P concentrations in control and fumigated plots. Fumigation reduced mycorrhizal infection and soil microbial biomass. Mycorrhizal infection of corn shortly after germination appears to be important to intitial corn growth in this soil. Fumigating soil revealed a considerable biological influence on corn growth and yield by reducing both deleterious and beneficial microorganisms.