Look at ?paste
for (j in 1:10) {
write.table(j, file=paste("haplo.txt", j, sep="."),
row.names=F, col.names=F, append=F, quote=F)
}
BTW, there have been many similar posts like this in the past. They are
easily found using the search function at
http://cran.r-project.org/search.html
HTH, Andy