Thanks a lot, this I(xx^2) ... worked. I guess, I should learn more abot the function poly itself. (so will I... :) ) Thanks again! -- View this message in context: http://r.789695.n4.nabble.com/non-linear-regression-for-3D-data-tp2320982p2325911.html Sent from the R help mailing list archive at Nabble.com.