Among passerines not nesting in cavities, mixed maternity within a clutch is rare and has been recorded at <1% of the nests of four North American species. Using genetic methods, we report the first case of mixed maternity in the Lark Sparrow (Chondestes grammacus). Within an unusually large clutch in Texas, we found that the attending female shared the mitochondrial haplotype with an unhatched egg and one nestling, but not with another nestling. Analysis with microsatellite DNA confirmed mixed maternity at the nest; the female and a nestling did not share alleles at five of nine loci analyzed. Various behaviors may lead to mixed maternity, including intraspecific reuse of nests, conspecific usurpation, and conspecific parasitism. Our study stresses the important roles that intensive nest monitoring and genetic tests can play in detecting cryptic reproductive strategies.