Dears UseRs, In Zuur's book (Mixed effects models and extensions in ecology with R), p 167, it is mentionned that the Variogram function (package nlme) scale the sill to 1. Here is the plot of the semi-variogram given by the Variogram function on my "lme" object. (http://imm.io/3OLe) Note that the semi-variance exceed 1 !! It can probably explain why the Gaussian correlation structure fitted in my "lme" model is strange (curve is probably fitted to the data above 1). See my previous message a few minutes ago untitled "Variogram (nlme) of a lme object - corSpatial element question." I don't know if it is a bug, but Implications could be important for model fitted with a spatial correlation structure. Arnaud [[alternative HTML version deleted]]