We have examined transgene inheritance in over 300 progeny of a line of soybean (Glycine max ) transformed by particle bombardment with a construct containing bovine b-casein under the control of the soybean lectin 5 0 and 3 0 regulatory elements. Four copies of the casein transgene, located at a single locus, exhibit a high frequency of recombination that resulted in novel patterns in approximately 16% of the progeny in both the T 1 and T 2 generations. Characterization of the transgene locus using restriction enzymes that do not cut the transformation plasmid showed that all four transgene copies are at a single locus no larger than approximately 40 kb in size. Therefore, the recombination events resulting in the loss of transgene DNA are taking place within a limited physical distance on the host chromosome. This is the first report extensively documenting transgene instability at the DNA level in a plant transformed via particle bombardment. As this report indicates, a seemingly simple phenotype (presence of the foreign protein) may conceal inherent genetic instability at the DNA level.