We demonstrate a mid-IR semiconductor laser-based absorption spectrometer for field measurements of ambient CH4. The field sensor uses a type II quantum cascade laser (or interband cascade laser, ICL) operating near 3.3 µm to monitor a well-isolated line in the 3 fundamental band of CH4. The ICL operates in cw mode at cryogenic temperatures. The sensor uses a multipass cell that provides an optical path of ~7 m with a 0.25-m base path. Thermoelectrically cooled InAs detectors are used along with balanced ratiometric detection to achieve a precision of 15 ppbv for a 60-s integration time. Several successful field demonstrations are carried out at sites maintained by the University of New Hampshire.