This paper examines bias toward status-similarity in adult friendships in Detroit and a West German city. Principles of meeting and mating, by which strangers are converted to acquaintances and acquaintances to friends, are stated. One of these, the proximity principle, claims that the more similar people are, the more likely they will meet and become friends. This principle is tested in matrices of friendship choice for twelve social characteristics. Two statistical measures of bias are used (odds ratio and marginal ratio) and their properties discussed. Compared to a random-choice model, adult friendships show strong bias toward status similarity for all social characteristics. Bias is strongest for edge categories of ranked statuses and for best friends. The less similar two people are in social characteristics, the less likely they are to be close friends. Demographic characteristics tend to show more bias toward homogeneous choices than other characteristics. These findings are explained and further analyses of adult friendship structure and dynamics are discussed. Urban research has shown that friendship ties among adults are pervasive and active, particularly when compared to neighbor and coworker ties (Axelrod; Bell and Boat; Bell and Force; Jitodai; Tomeh, a, b). These friendships are sources of social contact, intimacy, and assistance. They are considered primary ties, because of their emphasis on face-to-face contact and positive affect (Cooley; K. Davis; Parsons and Shils). Despite friendship's prevalence and assumed importance, little is known about the social structure of adult friendships. Who is chosen? What goods and services are exchanged? How large and heterogeneous are friendship networks? In this paper, one feature of these primary ties is examined: the structure of friendship choice. Most studies of friendship choice involve children or young adults, often college students (Lindzey and Byrne). There are relatively few reports of friendship choice by adults (Barnes; Berkun and Meeland; Curtis; Ellis; Gans, a, b; Greer; Kahl and Davis; Laumann, a, b; Lazarsfeld and *I thank E. 0. Laumann and F. U. Pappi for use of the Detroit and Altneustadt data sets. Laumann was principal investigator of the Detroit study; Laumann and Pappi, of the Altneustadt study.