Soils of the groundnut(Arachis hypogaea) L. basin of Senegal are impoverished with low fertility and organic matter content. Previously, farmers maintained yields of millet (Pennisetum glaucum(L.) R. Br. and groundnut with subsidized inorganic fertilizers and fallow periods to restore soils. Fish meal or by-product in various forms is a commercial commodity worldwide, but in Senegal the non-edible portion of fish (intestines, bones, scales and gills) are often discarded after processing. A three-year study was conducted on processed fish by-products as a soil amendment for millet and groundnut productivity. Chemical analysis showed that the processed fish byproducts are high in major nutrients such as N, 5.35%; P, 4.17%; K, 0.92%; Ca, 9.77%; and Mg, 0.36%. Consequently, the application of the processed fish by-products significantly (p < 0.05) increased millet grain from 0.29 Mg ha−1in the control plot (no fish by-product) to 2.50 Mg ha−1with 6 Mg ha−1fish by-product. Millet stover yield was also increased. Groundnut yields increased (p < 0.01) from 0.23 Mg ha−1in the control plots to about 1.00 Mg ha−1with 2 Mg ha−1fish by-product. Response curves for millet and groundnut were curvilinear with the incorporated fish remains explaining 98 and 99% of variability in yields of millet and groundnut respectively. Residual effect of fish by-product after one year significantly (p < 0.05) improved yields of millet and groundnut compared with inorganic fertilizer and equalled yields of the same crops with inorganic fertilizer after two years. However, stability analysis indicated that millet yields with fish by-product were less stable (s.e. = 0.31) than yields with inorganic fertilizer (s.e. = 0.16) reflecting the non-uniformity of the by-products and processing methods across villages.