Intraguild predation (IGP) is a mechanism that may facilitate the co-existence of native species with non-native invasive species. We conducted laboratory predation trials to assess the role of predator gape-limitation in the context of IGP between the endangered Mohave tui chub (Siphateles bicolor mohavensis) and invasive western mosquitofish (Gambusia affinis). Larval tui chubs had significantly lower (χ2 = 74.74; P < 0.001) survival in the presence of female mosquitofish (10.0%) than in the presence of male mosquitofish (73.3%). Reciprocally, adult tui chubs preyed upon adult mosquitofish, causing a significantly lower (χ2 = 11.33; P < 0.001) survival for male mosquitofish (60%) compared to female mosquitofish survival (96.7%). Vulnerability modelling revealed that mosquitofish with a body depth < 4.6 mm and a larval tui chub with a body depth < 1.2 mm were completely vulnerable to predation by adult Mohave tui chub and adult mosquitofish, respectively. IGP in this study system is size-structured based on gape-size limitation and may have some conservation implications for the recovery of endangered Mohave tui chub. Our findings also provide an important caveat to the dogmatic view of mosquitofish as a threat whenever they invade. It is important to note that many previous studies that reported negative impacts of mosquitofish involved native species with relatively small body sizes, often the same size as mosquitofish.