The matrix elements describing nonlinear effects in a weakly unstable plasma are interpreted by means of Feynman diagrams involving the shielded Coulomb potential, given byVs(q, &ohgr;) = 4&pgr;e2/q2&egr;(q,&ohgr;), where&egr;(q, &ohgr;)is the dielectric response function. The quasiparticles or plasmons are obtained from the poles ofVs, but effects of the nonresonant parts ofVsare also retained. All the matrix elements describing interactions of Langmuir oscillations in a strong magnetic field through second order in the quasiparticle numbers are found explicitly and shown to yield the correct classical results in the limitℏ → 0. In particular, the equations of Walters and Harris for three‐wave scattering in a cold plasma are extended to finite temperatures, and the shielding or dressing of the electrons is included in the equations for the nonlinear Landau damping. The self‐energy of a plasmon is also represented by diagrams and the optical theorem is used to compare the self‐energy with the scattering matrices. It is found that the quantum method provides a relatively simple way of deriving and interpreting equations for the time development of the wave spectrum and particle diffusion.