Questions surrounding musicians’ efficacy beliefs have received a growing amount of scholarly attention over the past decade. Most of such research has centered on self-efficacy belief, or one’s perceived competency for music-related tasks. Studies of collective efficacy perceptions among ensembles have been far fewer. None of the musical efficacy research I identified included investigations of within-group agreement, or the extent to which an ensemble’s members agree in their estimations of group capability. As explained by Bandura (1997), efficacy beliefs motivate both individuals and groups to pursue certain goals, invest effort, and persist despite challenges or setbacks. Thus, Bandura has argued, those with stronger efficacy beliefs are more likely to achieve success. In this exploratory study, I examined collective efficacy beliefs among collegiate string chamber music ensembles, including levels of within-group agreement and correlation with performance quality. Participants included 70 musicians representing 18 chamber ensembles from various collegiate and summer programs in the Northwestern (55 musicians from 14 ensembles) and Western (15 musicians from 4 ensembles) regions of the United States. Musicians independently completed a 5-item survey gauging their confidence in their group’s performance abilities; each ensemble’s aggregated results represented its collective efficacy belief. Each ensemble further provided a video-recorded performance of an excerpt from their repertoire. A panel of four string specialists rated each performance, and their combined ratings constituted performance quality measures for each ensemble. Ensembles in this study reported moderately strong levels of collective efficacy belief (M = 75.27%, SD = 9.09%; Mdn = 76.84%). Levels of within-group agreement were also uniformly high (mean rwg(J) = .95, SD = .05; median rwg(J) = .97), and there was a significant, moderately strong correlation between collective efficacy belief and within-group agreement (rS = .67, p < .01). I found no significant correlation between ensembles’ collective efficacy belief and performance quality. Findings from this study indicate fertile ground for future investigations. Reliability estimates suggest that the collective efficacy measurement instrument is suitable for use with string chamber ensembles, and correlational findings reveal potential limitations to the theorized link between efficacy belief and performance quality in chamber music settings.