[R] extracting submatrix from a bigger one

[matteo]

Hi guys,
I'm a newby, so sorry for the easy question.

I have a matrix (459x28) in which a large number of observations are 
repeated (same placed sampled in different times).
One of the columns is refers to the ID of the place of sampling.
What I would like is to extract subset matrix for every point of sampling.

I can do it manually, e.g. x1<-data.frame(dataset[dataset$ID=="x1",])
but is it possible to write a script and let do it to R?
So i got n submatrix of the n ID found in the original columns.

Cheers

Matteo