There is an increasing concern about the preservation of genetic identity of conventional maize (Zea mays L.) and of distance required to segregate non-genetically modified (non-GM) from GM grain production since the introduction of Bacillus thuringiensis (Bt) and other transgenic events into commercial hybrids. Field experiments were conducted at three sites in Ottawa, Canada, for 3 yr to determine (i) the extent of cross-fertilization of a maize genotype by foreign pollen of neighboring hybrids and (ii) the practical distance required to isolate conventional maize hybrids from neighboring GM maize fields. At each site, yellow-kernel Bt maize was planted in the center (27 by 27 m) of a field surrounded in all directions by the distance equivalent to 24 or 48 rows (37 m) of white-kernel maize, and a 200-m non-maize crop was maintained in all directions. Phenology and weather conditions were closely monitored during the tasseling and silking period. At maturity, a thorough examination on the cross-fertilization was conducted in the white maize population. Our results showed that the rate of cross-fertilization in maize was dependent upon the distance from the pollen source, wind direction and synchronization of silking and pollen shedding of the two genotypes involved. Up to 82% out-cross was measured in the first row adjacent to the Bt maize. The level of out-cross was <1% beyond the 37th border row (28 m) downwind and the 13th row (10 m) upwind in all site-years. An exponential decline model was fitted well (P < 0.01) to the cross-fertilization data as a function of distance from the yellow maize pollen source with R2 up to 0.64. Our data suggested that it is possible to produce non-GM maize grains by removing the outside rows of non-GM maize plants (about 30 m) neighboring the GM maize field in concern if the acceptance level is set at 1% out-cross. The generally recommended 200-m distance between two genotypes (inbreds, populations, hybrids, and wild relatives) appears to be appropriate for Bt or other GM maize, as well. Abbreviations: Bt, Bacillus thuringiensis ? CHU, crop heat unit ? GM, genetically modified