The Bmp signalling pathway is required for proper tissue patterning and formation during embryogenesis, but because it is essential for early development, it has been very difficult to investigate its specific functions at later stages. To circumvent this problem, Pyati et al. (see p. 2333) generated transgenic zebrafish containing an inducible dominant-negative Bmp receptor with which they inactivated Bmp signalling at selected time points in development. Their results show that during early gastrulation, Bmp signalling is important for tail organizer formation and for patterning the ventral mesoderm. However, from mid-gastrulation to early somitogenesis, the role of Bmp signalling changes – at these stages, Bmp signalling promotes ventral fin formation and, unexpectedly, prevents additional tails from forming. These findings thus reveal a specific late requirement for Bmp signalling during tail patterning, and highlight the dynamic temporal roles that this pathway plays during development.