Major Findings: The additional maize produced from a given amount of fertilizer applied varied widely across households even after largely controlling for soil and rainfall conditions. The median estimated response rate was 15.9kgs of maize per kg nitrogen applied; Under the range of conditions and smallholder management practices, average maize fertilizer response rates declined as the application rate increased beyond 2 bags of urea and 2 bags of D compound; Factors raising the response rate and profitability of fertilizer use included timely availability, application rates less than the MOA 4x4 recommendation, use of animal draft power in land preparation, and use of hybrid seed. In remote areas, and given current management practices, fertilizer use appears to be profitable only for a minority of smallholder farmers in the relatively remote areas. For farmers in the more accessible areas, profitability of fertilizer use depends on timely availability. If fertilizer is not available on time, even farmers in the more accessible parts of this area of relatively high agronomic suitability for maize production are largely unable to use fertilizer profitably. Because over 30 percent of the households reported that fertilizer was delivered late, these findings indicate that efforts to ensure timely distribution can contribute substantially to the productivity gains achievable from fertilizer use.