Success of supported exercise programmes to tackle obesity appear to be shaped, in part, by co-exercisers’ beliefs. This study, therefore, aimed to assess implicit attitudes towards obesity among two key groups of people in a public exercise setting: fitness professionals offering exercise advice, and regular exercisers.Questionnaire survey.In all, 57 fitness professionals and 56 regular exercisers were recruited from gyms across Central England. Participants completed a demographic questionnaire, semantic differential measure of explicit beliefs and the implicit associations test (IAT). The IAT reveals unconscious attitudes of participants to implicit associations between target concepts (thin vs. fat) and attributes (good vs. bad). The attribute of motivated vs. lazy was adopted in the current study due to relevance in an exercise setting.Evidence of a strong anti-fat bias was found (p<.01) for both fitness professionals and regular exercisers on all implicit and explicit measures (good vs. bad; motivated vs. lazy). This bias was more pronounced for fitness professionals who themselves had never been overweight and who believed personal control dictated body weight. For regular exercisers, a higher level of anti-fat bias was found for females, younger participants and those who had never been overweight.This study suggests that the guidance to support exercise, and combat obesity, may be compromised by the beliefs of those facilitating such programmes.