In this paper, the relation between the apparently stable or marginally stable electron reduced distribution and the excitation of Langmuir waves is discussed. It is found that the positive slope in the reduced distribution function is not always necessary for the excitation of Langmuir waves and that under certain circumstances, Langmuir waves propagating in oblique directions with respect to the ambient magnetic field can be excited by the electrons which may not have a bump in the reduced distribution. ¿ 2000 American Geophysical Union