 Thank you for your reply, Henrik. But if you see the attached document,
 you'll see there are no missing values at all, actually, I create 
specific files for each of the dependant variables I analyse, removing 
the missing values. 
   I have tried it, though, and I still get the error message, as expected.

   Thank you again! Let's see if someone else can help me.

   Iker 

--- El jue, 13/1/11, Henrik Thurfjell <Henrik.Thurfjell@slu.se> escribió:

De: Henrik Thurfjell <Henrik.Thurfjell@slu.se>
Asunto: RE: R-sig-mixed-models Digest, Vol 49, Issue 21
Para: "'karraspito@yahoo.es'" <karraspito@yahoo.es>
Fecha: jueves, 13 de enero, 2011 09:13

You may have NAs in your data, try na.action=na.omit and see if it works


--- El mié, 12/1/11, Iker Vaquero Alba <karraspito@yahoo.es> escribió:

De: Iker Vaquero Alba <karraspito@yahoo.es>
Asunto: [R-sig-ME] Matrix not symmetric
Para: r-sig-mixed-models@r-project.org
Fecha: miércoles, 12 de enero, 2011 23:20



   Hello all:



   My name is Iker. I have been looking at several R-help files online because of a 
problem I am experiencing when I try to fit one of the statistical 
models I need to analyse my data, but actually I have not found any 
satisfying solution, so I am writing just in case you can 
possibly give me a better solution. The model I try to fit is:



   
fledgecoltailmodel1<-lmer(nfledge~sex*briventral*inslarge*weatherpc1*tlength-sex:briventral:inslarge:weatherpc1:tlength-sex:briventral:inslarge:weatherpc1-sex:briventral:inslarge:tlength-sex:briventral:weatherpc1:tlength-sex:inslarge:weatherpc1:tlength-briventral:inslarge:weatherpc1:tlength-sex:briventral:inslarge-sex:briventral:weatherpc1-sex:briventral:tlength-sex:inslarge:weatherpc1-sex:inslarge:tlength-sex:weatherpc1:tlength-briventral:inslarge:weatherpc1-briventral:inslarge:tlength-briventral:weatherpc1:tlength-inslarge:weatherpc1:tlength+(site|pair),family=poisson)



   Obviously, the exaggerated length is due to the fact that, after 
writing the model, I remove all the interactions that are biologically 
impossible to interpret for me. "nfledge" is the number of chicks 
fledged in several barn swallow nests all over my field sites, so I 
interpret it as count data, that's why I use the "poisson" family for 
error distribution. when I type "summary(fledgecoltailmodel1)", I get 
this error message:



   Error in asMethod(object) : matrix is not symmetric [1,2]



   I cannot really understand what that means. As an interesting fact (I
 think), when I fit exactly the same model but without specifying the 
error family (i.e. assuming Gaussian error distribution) everything is 
OK and I get no error message. I wonder if I should go on with that, but
 my dependant variable is definitely count data. 

   

   According to the reply to a post of a girl called Luisa, which was experiencing a similar 
problem, I tried updating my "lme4" package, and I didn't load the 
"nlme" one together with "lme4", but I am still getting the same error 
message.



   I would very much appreciate if you could have a look at my problem 
and try to help me. Thank you very much in advance for your help.



   Iker Vaquero-Alba

   Centre for Ecology
and Conservation

   Daphne du Maurier
Building

   University of Exeter,
Cornwall Campus

   Treliever Road

   TR10 9EZ Penryn

   U.K.


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