If a fellow converted (his children remained gentiles) and prior to his conversion he had children - has he fulfilled the Mitzva of Peru U'Revu? (sources)
This is part of a discussion on Yevamot 62a. The Shulchan Aruch rules (EH 1:7) that if he and his children converted he has fulfilled his mitzva. The Chelkat Mechokek there (sk 9) quotes the Marharil (#223) that even if his children do not convert he has fulfilled his mitzva.