TH-stopping, reported in the speech of working-class and immigrant groups across the U.S., has long been considered a regional feature of New York City English (NYCE). Its origins in NYCE have been anecdotally attributed to the non-native speech of the first immigrants to the area, such as the Irish, Italians, and Poles (Babbitt 1896, Labov 1966). This paper seeks to provide conceptual and acoustic evidence for substrate origins of TH-stopping in one ethnic community in New York City. I analyze interdental fricatives produced by bilingual Polish Americans who were born in NYC (Generation 2) or have resided there since their early teens (Generation 1). An acoustic analysis of underlying and “substituted” stops reveals that the latter employ the Polish voicing contrast. Stopping rates are also found to vary according to style, generation, and gender. Specifically, TH-stopping is favored in sociolinguistic interviews (relative to reading tasks), and in the speech of first generation men and second generation women. Lastly, speakers’ cultural orientation and use of Polish correlate strongly with stopping rates. Taken together, these results suggest that TH-stopping in the Polish community did originate as a substrate effect, but has since developed into a female-led ethnic marker.