In mice, the formation of lymphatic vessels (lymphangiogenesis) requires the homeodomain transcription factor Prox1. Here, Stefan Schulte-Merker and colleagues examine whether the role of Prox1 is conserved in zebrafish (p. 1228). Using a novel transgenic reporter line, the researchers show that, in contrast to the situation seen in mice, zebrafish Prox1 is initially not expressed in all lymphatic precursor cells and reliably marks this population only during later stages of lymphangiogenesis, arguing against a role for Prox1 in lymphatic specification. In addition, targeted mutagenesis demonstrates that lymphangiogenesis can proceed in the complete absence of Prox1. Finally, they show that the functionally related transcription factors Coup-TFII and Sox18, which are implicated in lymphatic specification in mice, are also dispensable for zebrafish lymphangiogenesis. The authors conclude that an alternative lymphatic specification mechanism is present in zebrafish and propose that differences in the timing of lymphangiogenesis between mice and fish can explain this divergence.