ErCoSi2 crystallizes in the orthorhombic CeNiSi2-type structure with t
he Cmcm space group and orders antiferromagnetically near T(N) = 2.6 K
. The magnetic structure is incommensurate with a wavevector k = (0.46
7, 0, 0.5). The magnetic moment is in the a-c plane. The temperature d
ependence of lattice constants indicates a positive exchange striction
. The symmetry analysis of the incommensurate magnetic structure gives
the best agreement with experimental data.