a <-data.frame(a=c("a","a","b","c","b","d"),b=c(1,2,3,4,2,3))
> a
  a b
1 a 1
2 a 2
3 b 3
4 c 4
5 b 2
6 d 3

a b
1 a 2
2 b 3
3 b 2

a[which(a[,"a"]=="a"),]