The paper describes results of experiments conducted to investigate the kinetics of in vitro competition between natural progenitor Autographa californica (Acv E-2) and the recombinant Ac360-Bgal virus strains. The following conclusions can be drawn from the analysis: Selection pressure appeared to be fairly constant throughout the experiment; Selection was virtually unidirectional in favor of the natural strain regardless of the starting ratio, and consequently, the natural strain eventually predominated; The observed high frequency of mixed phenotype plaques appear to be influenced by some combinational phenomenon other than recombination at the DNA level; and The frequency of mixed phenotype plaques are also affected by passage. All evidence in the experiment points toward a disfavored recombinant genotype which appears not to be capable of competing effectively in vitro. The data present information that can be utilized as a basis for assessing the potential health effects of baculovirus expression vectors.