Mallards (Anas platyrhynchos) were virtually absent from central Ontario as recently as 1948 but now exceed 60 pairs/100 km2 in some areas. Black Duck (A. rubripes) numbers in central Ontario appear stable with breeding densities in some areas exceeding 40 pairs/100 km2. Densities of breeding Mallards and Black Ducks exhibit spatial variability suggesting that habitat quality may influence distributions. We used data from Canadian Wildlife Service breeding pair surveys to classify wetlands as used only by breeding Mallards, used only by breeding Black Ducks, shared by Mallards and Black Ducks, or vacant (i.e., not used by either species). We evaluated wetland fertility by analyzing water chemistry (color, pH, conductivity, alkalinity [CaCO3], calcium, magnesium, sodium, potassium, total phosphorus) and physical characteristics (shoreline irregularity index [SI], percent open water, wetland size) for 447 wetlands. Wetlands that Mallards shared with Black Ducks or solely occupied were the most fertile. Vacant wetlands were least fertile. Areas that supported few Mallards or Black Ducks had relatively infertile wetlands; Mallards predominated in areas with relatively fertile wetlands. We conclude that wetland fertility has influenced the distribution of Mallards and Black Ducks in central Ontario. Mallards and Black Ducks apparently prefer wetlands with similar characteristics (i.e., high fertility, moderate open water, high SI, and small size), so competition for breeding sites is likely. Historical data from Ontario lead us to suspect that this competition may have contributed to the decline in Black Duck numbers.