treat_code isn't a factor, but a numeric variable. You should use: summary(aov(EtoH~as.factor(treat_code), mydata)) TukeyHSD(aov(EtoH~as.factor(treat_code), mydata)) Bart -- View this message in context: http://n4.nabble.com/TukeyHSD-troubles-tp1570205p1570228.html Sent from the R help mailing list archive at Nabble.com.