The first monolithic integration of an interferometric modulator with a gain-coupled distributed-feedback laser is reported. The integrated MZ/DFB is fabricated by butt-coupling in order to minimise design constraints on the MZ and DFB active regions. 10Gbit/s transmission over 100 km of NDSF at lambda = 1.55 mu m is achieved with a receiver penalty of <1dB. similar to 90% of the integrated lasers emit in a single longitudinal mode.