A (probably) trivial question about first occurrence in unique()

[Sarah Henderson <sarah.henderson@ubc.ca>]

Hello to all --

I have data for 300000 road segments reflecting approximately 60000 
unique street names.  I would like to create a new data frame with 
the unique street names and their first-occurrence municipalities 
(they are sorted North to South).  Of course unique(STREETS) gives me 
the first part, but is there some way to get the MUNIC of the first 
occurrence without writing a loop?
Many thanks,

Sarah