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Animals are believed to be homeostatic by maintaining relatively constant body nutrient content independently of changes in diet. Phosphorus (P) is one of the most important elements for fishes as Loricariids, which are covered with armor-like bony plates. These species are expected to maintain nutrient homeostasis, however, in environments with low P availability they can be P-limited. The hypothesis of this study is that P concentration in body composition of an herbivorous fish, Hypostomus jaguribensis, does not change in environments with different availability of this nutrient. We conducted this study in two locations of the Curu river basin in Brazil, which differed in their nutrient concentrations: one was ultraoligotrophic and the other mesotrophic, as determined by total phosphorous concentrations and chlorophyll a in the water. We found significant differences in the body P content of the fish from the two sites: Hypostomus jaguribensis from the ultraoligotrophic site showed higher body P content, higher body weight and condition factor. This suggests that the ultraoligotrophic site is a more favorable environment for this species. The body P content was higher in intermediate sizes (between 12 and 14 cm in the ultraoligotrophic site and between 11 and 13 cm in the mesotrophic site), which can be related to biological characteristics such as the need to accumulate nutrients at the beginning of the reproductive period. Our study did not find support for strict homeostasis in this high-P demand fish species.