Primate diets in captivity often differ considerably in their nutrient composition to those eaten by wild conspecifics. In particular, captive diets often contain much higher levels of sugar and other readily-digestible non-structural carbohydrates and much lower levels of fibre. This has been shown to have negative effects on captive primate physical health but to date there is little evidence of any effects on behaviour. In line with ongoing dietary improvements the diets of four species of lemur housed at Newquay Zoo and Paignton Zoo were changed to completely remove all fruit, resulting in a lower concentration of non-structural carbohydrate and increased fibre, to better reflect the composition of their wild diet. The effects of this diet change on behaviour of the lemurs were monitored, paying particular attention to possible welfare indicators: aggression, auto-grooming, foraging and self-directed behaviour. When fed the fruit-free diet both aggression (p < 0.001) and self-directed behaviour (p < 0.001) were significantly lower than when fed the original diet in all four lemur species. There was no significant effect of diet on foraging and auto-grooming. These results suggest that feeding a fruit-free diet for these lemur species has a positive effect on their psychological welfare in a zoo setting.