A negative aspect of glyphosate-based management systems is the potential for volunteer glyphosate-resistant (GR) crop plants to impact subsequent crops. Studies were conducted to evaluate density and duration of interference effects of GR soybean (Glycine max (L.) Merr.) on GR cotton (Gossypium hirsutum L.) growth and yield. Our study demonstrated that regardless of soybean density and duration of interference, soybean did not affect cotton height at harvest. Season-long interference with a soybean density of 1 plant per row m would be expected to reduce cotton yield 14%. A cotton yield reduction of 0.4 to 1% can be expected with only 1 week of soybean interference at a density of 5.25 plants per row m. Expected yield reductions of 1 to 3, 5 to 6, and 8 to 11% would be observed with soybean interference for 2, 4, and 6 weeks, respectively. Results demonstrate the sensitivity of cotton yield to soybean interference, indicating that soybean can be considered a problematic weed in cotton necessitating early management.