Using combined Er$sup 3+$ absorption spectroscopy and bulk magnetization measurements, the magnetic properties of ErCrO$sub 3$ in the GAMMA$sub 1$, GAMMA$sub 2$, and GAMMA$sub 4$ phases have been studied as a function of temperature and external field. The experimental data are analyzed using a single-ion effective field model. For the GAMMA$sub 4$ and GAMMA$sub 1$ phases the most general Cr-Er and Er-Er coupling terms allowed by symmetry are considered and the canting angle of the ordered Cr$sup 3+$ spins (in the GAMMA$sub 4$ phase) is not constrained a priori to be temperature independent. It is concluded that the Cr-Er coupling is antisymmetric in nature and due mainly to Dzyaloshinsky-Moriya type exchange. For this type of interaction, the Cr$sup 3+$ canting angle is indeed temperature independent and this conclusion justifies the constant-canting-angle assumption used extensively in the literature. The Er- Er coupling is found to be of dipolar origin and it is shown that this interaction results in a significant temperature dependence of the effective splitting factor as deduced from optical-absorption measurements. For the GAMMA$sub 2$ phase, the data for temperatures above 20 degreeK are interpreted in terms of a constant-canting-angle model for the Cr-Er interaction. At lower temperatures it ismore » argued that the dipolar Er-Er coupling must also be considered. (AIP)« less