Different interactions of two common phenomena--final consonant omission and vowel lengthening before voiced consonants--are examined with a focus on a case study of two young children with phonological delays in their acquisition of English. Argues that at least some developmental opacity effects support sympathy and that such effects emerge in the course of development from the harmonic ranking of sympathy over input-output faithfulness and the incremental demotion of markedness constraints. (Author/VWL)