Summary: A new hypoplastic equation is proposed and its capacity of representing the mechanical behaviour of a silty soil is examined. In this constitutive equation a particular soil is characterised by four material constants having a clear physical meaning (three of them are standard in soil mechanics), which makes the calibration procedure and the interpretation of results easier. Its performance in the prediction of results of several laboratory tests (especially triaxial tests along different stress paths) and as compared to Cam–Clay predictions is, in general, good. The nonexistence of an elastic domain in hypoplasticity provides a more realistic response than that of Cam–Clay in stress paths in which the octahedral stress is reduced. The equation is able to reproduce important characteristics of soils as critical states, the normalised behaviour, the behaviour in proportional deformations, the behaviour in different stress paths. Besides, the equation is simple enough to permit a comprehensive analysis of its response. This is done with the help of the theory of systems of ordinary differential equations. Main results are analytical solutions in proportional deformations and the response diagram.