This qualitative study explored the social experiences and HIV-related sexual practices of 30 young gay and bisexual men who moved to New York City in the past 3 years from other countries or elsewhere in the United States. For many migrants, a key basis of vulnerability to HIV was their engagement with New York City's unfamiliar sexual culture. Many recent arrivals migrated from places with small gay communities and low HIV prevalence, and some came with a practice of limited condom use. Participants described encountering an abundance of sexual opportunity in New York City, accessible to even the newest arrivals through internet sex sites. Some migrants expressed surprise that few men they met were interested in dating or establishing trust before having sex. Although frequent HIV testing was common, HIV status, testing history, and condom use were seldom discussed with sex partners for some men even with new partners or before sex without condoms. International and in-country migrants who are beginning to navigate New York City's gay sexual culture may be more vulnerable to HIV infection than established residents if they are inexperienced in encountering vast sexual opportunity, are less practiced in local norms of sexual communication, or if their lack of economic resources or social connections encourages them to have sex for money or shelter. This article suggests HIV prevention interventions for urban migrants and other men who have sex with men.