Variation in 33P incorporation in acid-soluble (AS), nucleic acid (NA) and phospholipid-phosphoprotein (PL) phosphate fractions in roots were investigated for three families of slash pine (Pinus elliottii) under five N/P fertilizer regimes. Height and weight growth and P accumulation as acid soluble P in shoot terminals were also characterized for comparisons. Phosphate incorporation was largely determined by the P status of the seedlings as higher levels were assayed in organic fractions of seedlings under low P than when grown with high P. Both N level and family also affected incorporation. Interactions suggested that some families accumulate P differentially under various fertilizer treatment regimes. Certain families also translocated more P to shoots. Highest P incorporation in shoot terminals was under the N50P50 treatment. Families that accumulated more P in roots generally showed higher P activities in the shoot terminal. Families also interacted with fertilizer treatment in this measure. Incorporation of P in the acid-soluble fraction of shoot terminals was closely correlated with height growth and top fresh weight. This study indicates that in slash pine seedlings early selection for phosphate incorporation into metabolic fractions can be conducted as a correlated response to growth. Because of the effect of fertility regime and the interactions of these with families, selections should be conducted under nutrient regimes typical of the expected growth medium of the trees. Forest Sci. 22:21-30.