Since 1988, cucurbit crops, particularly watermelon, cantaloupe, and squash, grown in Oklahoma and Texas have experienced devastating losses from cucurbit yellow vine disease (CYVD), caused by the phloem-limited bacterium Serratia marcescens Bizio. Squash bug, Anasa tristis (De Geer), is a putative vector of the pathogen. In 2000–2001, overwintering populations of squash bug collected from DeLeon, TX, were tested for their ability to harbor and transmit the bacterium. Individual squash bugs (n = 73) were caged serially for periods of up to 7 d on at least four squash seedlings. Two studies were conducted, one with insects collected in November 2000 placed on first true leaf-stage seedlings and the second with insects from an April 2001 collection, placed on 3–5 true leaf-stage squash. Controls consisted of squash seedlings caged without insects. Squash bug transmission rates of the pathogen in studies I and II were 20 and 7.5%, respectively. Overall, 11.0% of the squash bugs harbored and successfully transmitted the bacterium to squash seedlings. All control plants tested negative for S. marcescens and did not exhibit CYVD. Female squash bugs killed a significantly greater proportion of young first leaf-stage seedlings than males. Feeding on 3–5 leafstage squash resulted in no plant mortality regardless of squash bug gender. This study demonstrated that the squash bug harbors S. marcescens in its overwintering state. The squash bug-S. marcescens overwintering relationship reported herein greatly elevates the pest status of squash bug and places more importance on development of integrated strategies for reducing potential overwintering and emerging squash bug populations.