According to the nativist interpretation of Optimality Theory (Prince and Smolensky (1993)), all language learners are born with a set of universal constraints. Learnability arguments entail that, in the initial state, markedness constraints outrank faithfulness constraints (Prince (personal communication, September 26, 1993), Smolensky (1996a), Tesar and Smolensky (2000)). This study investigates whether English learners give evidence of observing markedness and faithfulness constraints relating to nasal place assimilation. Employing the Headturn Preference Procedure (Jusczyk (1998b), Kemler Nelson et al. (1995)), these investigations introduce a general experimental paradigm for exploring infants' phonological grammars. English learners at 4.5 and 10 months of age gave evidence of observing both markedness and faithfulness constraints and ranked markedness above faithfulness. After a brief instability around 15 months, at 20 months they display the adult English pattern, with markedness outranking faithfulness.