Identification of phosphorus (P)-responsive genes is important in diagnosing the adequacy of dietary P intake well before clinical symptoms arise. The mRNA abundance of selected genes was determined in the intestine, pyloric caeca, and kidney of rainbow trout fed low-P (LP) or sufficient-P (SP) diet for 2, 5 and 20d. The LP/SP ratio of mRNA abundance was used to evaluate the difference in gene expression between LP and SP-fish, and to compare the response to bone and serum P, which are conventional indicators of P status. The LP/SP ratio of intestinal, caecal and renal type-II sodiumphosphate cotransporter (NaPiII) mRNA abundance changed from ~1-2 (d2), to ~1.4-4 (d5) and to ~2-10 (d20). The LP/SP ratio of renal NaPiII, vitamin D 24-hydroxylase, and vitamin D-receptor mRNA abundance correlated inversely with serum P on d5, but not d2 and d20. In another study, differentially expressed genes between LP and SP-fish were examined by subtractive hybridization, confirmed by Northern blot, and evaluated by t-test and correlation with serum and bone P concentrations. About 30 genes were identified as dietary P-responsive at d20, including intestinal meprin and cysteinesulfinic acid decarboxylase, renal S100 calcium-binding protein and mitochondrial Pi carrier, and caecal apolipoprotein E, somatomedin B-related protein, and NaPiII. The LP/SP ratio of mRNA abundance of renal mitochondrial Pi carrier and intestinal cysteinesulfinic acid decarboxylase changed significantly by d2, and intestinal meprin by d5. Hence, these genes and NaPiII are among the earliest steady-response genes capable of predicting P deficiency well before the onset of clinical deficiency.