Blues pulses 300 fs long have been generated from 150 fs infrared pulses by compressing second harmonic radiated in the Cerenkov geometry. Mode‐locked Ti:sapphire pulses were prism coupled into a 1‐cm long proton‐exchanged MgO:LiNbO3planar waveguide. The 2 ps Cerenkov output, chirped due to group velocity mismatch, was then compressed by a diffraction grating down to 300 fs. The use of mode‐locked pulses results in a conversion efficiency of 1000%/W/cm, three orders of magnitude higher than by cw doubling in the same geometry. The Cerenkov geometry ensures that all wavelengths can be phase matched over long crystal length, so that even low power lasers can be efficiently doubled. The dispersion element ensures that the chirped output can be compressed into the femtosecond regime. This concept can be extended to femtosecond pulses in any two‐photon process in any waveguide material and any mode‐locked source. ©1995 American Institute of Physics.