Soil acidity is a major constraint to maize (Zea mays L.) productivity in tropical soils due to toxic levels of aluminium (Al) and phosphorus (P) deficiency.Selection for acid-soil tolerance is a sustainable way for increasing maize yields on these soils. The objectives of this study were to:(i) determine the genetic effects of maize traits associated with phosphorus efficiency (ii) compare the genetic effects of maize traits in low P and high P in acid soils. Six F1 single crosses derived from acid soil tolerant and susceptible lines were used in this study. The parental inbred lines, the F1’s, F2’s, BC1P1, BC1P2, from each of the six crosses were evaluatedin two low P acid soils of western Kenya. Mean genetic effect (m), additive genetic effects (a), dominant genetic effects (d) and epistatic digenic effects (aa, ad, dd) were computed for Grain yield (GYLD),Shoot dry matter (SDM), Root Length density (RLD), P content (PC), P utilization efficiency (PUE) and P efficiency (PE).For most of the traits, greater variation was accounted for by dominance followed by epistatic and additive genetic effects under both P conditions. Means for all the traits studied were significantly higher at high P conditions compared to low P ones for all the generations. SDM, RLD, PC and PE exhibited higher means under high P for all the generations. Mean heritabilities were generally higher athigh P although, dependent on the generation and the trait. Highest PE was exhibited by the F1s (59%) and lowest F2 (52%). The magnitude of both additive and non-additive gene effects were always greater in high P compared to Low P pointing to the possible effects of P variation on gene action. The results suggested that the genetic effects on major PE traits did not differ under different P regimes.