The developmental biology and fitness of the carob moth, Ectomyelois ceratoniae (Zeller), is highly variable and depends on many environmental factors. This study was conducted to determine the impact of fruit moisture content on carob moth mortality, development, and fitness. We found a significant relationship between fruit moisture content and carob moth mortality and development. Only 1 of 61 carob moths completed larval development within 85 d in fruit having <5.0% moisture content, and no larvae survived at moisture contents <3.5%, close to the predicted minimum of 3.2% moisture content; 75% of the larval mortality occurred below 7.3% fruit moisture. Of the 32 larvae that were still developing on day 85, 75% were below 13.2% fruit moisture. Average female development ranged from 49.3 ± 4.2 d at 21.8 ± 0.7% fruit moisture to 81.0 d at 5.0% fruit moisture, and average male development ranged from 47.7 ± 4.8 d at 21.8 ± 0.7% fruit moisture to 75.5 ± 4.5 d at 5.1 ± 0.3% fruit moisture content. Maximum predicted development occurred at 19.6 and 20.4% fruit moisture content for females and males, respectively. Female fitness was more adversely affected at low fruit moistures than was male fitness as indicated by a greater decrease in adult emergence weight and greater increase in developmental time. Adult longevity of both sexes was not influenced by the fruit moisture at which they were reared as larvae. The average number of eggs laid by females was linearly related to the fruit moisture at which they were reared as larvae. Population fitness as estimated by the population doubling times was negatively affected by low fruit moisture content, and times ranged from 9.9 to 210.4 d over the range of conditions in this study. Future studies aimed at testing fitness parameters of the carob moth should control fruit moisture content to make valid inferences concerning the impact of environmental factors on development and fitness of this species.