Development of ultrashort pulse sources has had an immense impact on condensed-matter physics, biomedical imaging, high-field physics, frequency metrology, telecommunications, nonlinear optics, and molecular spectroscopy. Although numerous advancements of such sources have been made, it remains a challenge to create a highly compact, robust platform capable of producing femtosecond pulses over a wide range of wavelengths, durations, and repetition rates. Recent observations of frequency comb generation via cascaded parametric oscillation in microresonators11 suggest a path for achieving this goal. Here we investigate the temporal and spectral properties of parametric combs generated in silicon-nitride microresonators and observe a transition to passive modelocking of the comb consistent with soliton-pulse formation, resulting in the generation of 160-fs pulses at a 99-GHz repetition rate. This platform offers the prospect of producing pulses from 10 fs to a few ps at repetition rates from 10 GHz to > 1 THz and over a wavelength range of 0.8 - 6 \mu m.