This study compares Arabic L1, L2, and heritage speakers' (HS) knowledge of plural formation, which involves concatenative and nonconcatenative modes of derivation. Ninety participants (divided equally among L1, L2, and heritage speakers) completed two oral tasks: a picture naming task (to measure proficiency) and a plural formation task. The findings indicate that both L2 learners and heritage speakers have consistent problems with nonconcatenative plural morphology (particularly plurals with geminated and defective roots). However, the difficulties that heritage speakers displayed were mainly restricted to forms that are acquired late by L1 children, unlike L2 learners who displayed a sharp performance dichotomy between concatenative and nonconcatenative plurals. Furthermore, with regard to the default strategy, heritage speakers resorted to the language-specific default form, namely the sound feminine, whereas L2 learners opted for the sound masculine, which is likely a case of adhering to a universal tendency.