Susceptibility measurements from dc to 500 kHz have been made on the linear coordination polymer, poly(cobalt n‐butylphosphinate). At high temperature the data conform to the classical Heisenberg antiferromagnetic chain with a mean field temperature of 28 K. Below about 20 K there is an excess susceptibility which saturates in modest fields. These results are described in terms of an antisymmetric exchange component which produces short‐range weak ferromagnetic order in the paramagnetic phase. At still lower temperature the ac susceptibility shows a loss component, even in zero applied field, which we ascribe to the motion of domain walls in the weak ferromagnetization.