Parametric decay processes have been studied using rf probes and CO2laser scattering during the lower‐hybrid wave heating and current drive experiments in the Alcator C tokamak. The most important process is believed to be the nonresonant decay into ion‐cyclotron quasimodes and/or that into electron‐Landau quasimodes. At lower densities &ohgr;0/&ohgr;lh(0)>2 [where &ohgr;0is the frequency of the injected pump wave and &ohgr;lh(0) is the lower‐hybrid frequency evaluated at the plasma center], where efficient current drive and electron heating are obtained, parametric decay is absent or very weak. At higher densities [&ohgr;0/&ohgr;lh(0) ≲2] strong parametric decay is observed which correlates well with ion tail formation near the plasma edge. Above these densities the electron tail is no longer formed and no significant heating or current drive have been obtained. Parametric decay may be responsible, at least partially, for loss of wave power near the plasma periphery.