Erosion of differential object marking (DOM)—the overt morphological marking of animate direct objects—has been observed in Spanish heritage speakers who are second-generation immigrants in the United States (Montrul 2004, Montrul & Bowles 2009). We investigated whether DOM is similarly vulnerable in heritage speakers of Hindi and Romanian, two other languages that also exhibit DOM, as well as in first-generation immigrants, adults who are presumably the main source of input to heritage speakers. We report the results of three experimental studies testing acceptability of DOM through a bimodal judgment task in first- and second-generation Spanish, Hindi, and Romanian speakers in the US and native speakers in Mexico, India, and Romania matched for age and socioeconomic status. Our results show structural changes with DOM in all of the heritage speaker groups to different degrees. Acceptance of nontarget DOM omission was more extensive in Spanish than in Hindi and Romanian. First-generation Hindi and Romanian immigrants did not differ in their grammatical proficiency and acceptance of DOM omission from the Hindi and Romanian speakers tested in India and in Romania. However, the first-generation Mexican immigrants displayed similar performance to the Spanish heritage speakers, suggesting that Spanish DOM is prone to L1 attrition in the first generation as well. We discuss linguistic and experiential factors relevant to the three languages and the three immigrant communities to explain these findings.