We consider a system of particles interacting via a screened Newtonian
potential and study phase transitions between homogeneous and inhomogeneous
states in the microcanonical and canonical ensembles. Like for other systems
with long-range interactions, we obtain a great diversity of microcanonical and
canonical phase transitions depending on the dimension of space and on the
importance of the screening length. We also consider a system of particles in
Newtonian interaction in the presence of a ``neutralizing background''. By a
proper interpretation of the parameters, our study describes (i)
self-gravitating systems in a cosmological setting, and (ii) chemotaxis of
bacterial populations in the original Keller-Segel model