Dispersion relations for ion-acoustic-like waves in plasmas with and withou
t superthermal particles are calculated using a simple pole expansion of th
e distribution function for the ions. It is found that slow weakly damped m
odes exist in two-temperature plasmas, and the conditions when these modes
appear are investigated. For plasmas with equal ion and electron temperatur
es weakly damped modes are found when there is a depletion of the high-ener
gy tails. (C) 2001 American Institute of Physics.