set.seed(42)
x <- data.frame( A = 1:10, B = 21:30 )
rownames( x ) <- sample( LETTERS, 10 )
x
## A B
## X 1 21
## Z 2 22
## G 3 23
## T 4 24
## O 5 25
## K 6 26
## V 7 27
## C 8 28
## L 9 29
## R 10 30
rownames(x)[3] #third row name
## [1] "G"
rownames(x)[x$B == 30]
## [1] "R"