Glyphosate is a broad-spectrum herbicide that ranks among the top ten herbicides used in the U.S. for agricultural and non-agricultural use. Glyphosate is sprayed on 100% of the crop area planted with genetically engineered crops resistant to RoundUpRTM such as soybean (RoundUpRTM Ready Soybeans).;It has been determined that glyphosate has fungicidal properties on some pathogenic and saprophytic fungi. Studying the interaction of glyphosate with entomopathogenic fungi is important because a wider use of this herbicide is predicted with glyphosate-tolerant crops. Under normal weather conditions, entomopathogenic fungi are considered the most important mortality factor of several arthropod pests in Iowa (insects and spider mites), maintaining their populations below economic thresholds.;The main objective of this research was to determine if outbreaks of twospotted spider mite (Tetranychus urticae Koch) and green cloverworm populations (Hypena scabra (F.)) could be expected with the use of transgenic glyphosate-resistant soybeans and their herbicides in commercial fields.;Laboratory studies determined that some RoundUpRTM formulations, and not the active ingredient (glyphosate), had fungicidal properties on four entomopathogenic fungi important in Iowa crops (Neozygites floridiana Weiser and Muma, Nomuraea rileyi (Farlow) Samson, Beauveria bassiana (Balsamo) Vuillemin, and Metarhizium anisopliae (Metchnikoff) Sorokin). Host suitability studies showed that green cloverworm does equally well developmentally, and possibly reproductively, when fed traditional varieties and glyphosate treated and non-treated varieties.;Field studies conducted in 1999 (Iowa, Minnesota and Ohio) and 2000 (Iowa) showed that in drought conditions, commercial fields planted with RoundUp RTM Ready Soybeans had higher spider mite populations than fields planted with traditional varieties. Occurrence of N. floridiana (entomopathogen of spider mites) was lower in fields planted with RoundUp RTM Ready Soybeans than with traditional varieties. No statistical differences were detected in green cloverworm populations or occurrence of its entomopathogenic fungus (N. rileyi) between transgenic and traditional varieties.;Results from these studies suggest that outbreaks of twospotted spider mite are more likely to occur on RoundUpRTM Ready Soybeans than on traditional varieties under environmental conditions suitable for spider mites.