The Northern Mockingbird (Mimus polyglottos), the state bird of Texas, Florida, Mississippi, Tennessee, and Arkansas, is noted for the highly complex and melodious springtime song of males. It is ideal for studying mate choice and breeding success because the flamboyant male song is surprising in a socially monogamous bird. I conducted a three year study of mating and breeding success to test the hypothesis that male song, measured as mean bout length and versatility, could be explained by high rates of extra-pair matings, something which has been found in many other apparently monogamous birds. However, I found that extra-pair fertilization accounted for only 3.1% of all young, so my initial hypothesis was disproved. Because song could not be explained by extra-pair fertilization, I investigated the role of this and other variables in mockingbird mate choice and breeding success. The additional variables included: four initial dates in the breeding season, prior-year experience, age, singing frequency, territory quality measured as amounts of shrubbery, three measures of size, three measures of white plumage areas, condition measured as subdermal fat amounts, and blood and gut parasitemia. I looked at the role of these characters in mate choice by comparing mated and unmated males in two years and found that amounts of white plumage, weight, wing lengths, first date sighted on territory, first date heard to sing, blood parasitemia, age, and territory quality were different (P < 0.10) in one or both years. In 1993, 26.1% of mated males were bigamists (1992: 6.5%) who differed from monogamous males in first date heard singing and blood parasitemia. Females prefer older, larger, healthier males with good territories. The following were predictors (P < 0.10) of breeding success in one or both years, measured as total numbers of broods or offspring, for either or both males and females: song variety, first date sighted on territory, first date heard singing, and first date building the first nest, prior-year breeding experience, prior-year territory occupancy, singing frequency, weight, wing length, fat levels, and territory quality. Experienced, healthier individuals with better territories and males with more song variety who sing less frequently have greater reproductive success.