Consistent, effective biological control of soilborne pathogens has been difficult to achieve in the field. Most research regarding biological control efficacy has focused on the biological control agent itself (i.e. dose, formulation, survival) or on its interaction with the rhizosphere and edaphic environments: Very little research has focused on how pathogen diversity affects the outcome. A dual-culture, in vitro assay was used to test the responses of 16 <i>Pythium</i> species (148 isolates) from three forest nurseries to <i>Streptomyces lydicus</i>. <i>Pythium </i>isolate responses (percent growth inhibition, inhibition zone distance, and mortality) to <i>S. lydicus</i> were measured and found to differ significantly among <i>Pythium</i> species and isolates. Furthermore, the responses of three species, <i>P. irregulare</i>, <i>P. sylvaticum</i>, and<i> P. ultimum</i>, were different depending on which nursery they were isolated from. Isolate growth rates strongly biased percent growth inhibition, and to a lesser extent, the inhibition zone distance. Results indicate that growth rate must be accounted for when conducting in vitro, dual-culture biological control inhibition assays and suggest that pathogen diversity deserves further exploration as a potential component contributing to inconsistent biological control.