Spontaneous sex reversal is a form of intersexuality in which one sex, usually a female, acquires traits of the other sex. We reviewed previously published accounts of intersex Mallards (Anas platyrhynchos) resulting from sex reversal, of which four individuals behaved as females and four behaved as males, and two of the latter mounted other females. Video analyses of an intersex Mallard in Napa, Napa County, California, revealed that it was less social, spending much more time alone than either sex of normal Mallards during the breeding season, and behaved like a female by spending much more time with males than with females. It is unknown why some intersex Mallards behave as females and others as males.