We introduce the concept of absolute differential gravimetry using fiber optic-based laser interferometry. Absolute differential gravimetry provides an instantaneous determination of the difference in gravity between two or more points on or beneath the surface of the earth. Applications include enhancing the resolution of the gravity field, as well as subsurface fluid and structure monitoring. With a network of sensors, observation of time dependent variations of the gravity field is also possible. Results from a proof-of-concept prototype instrument using fiber-optic coupler technology represent the first time absolute laser interferometric differential gravity data have been collected. Implications of the new measurement method in terms of geophysical studies and future research plans are discussed. ¿ 2000 American Geophysical Union