I have a data frame with two variables, Date and Taxa and want to get the date for the first time each taxa occurs. There are 9 different dates and 40 different taxa in the data frame consisting of 172 rows, but my answer should only have 40 rows. Taxa is a factor and Date is a date. For example, my data frame (called 'species') is set up like this: Date Taxa 2013-07-12 A 2011-08-31 B 2012-09-06 C 2012-05-17 A 2013-07-12 C 2012-09-07 B and I would be looking for an answer like this: Date Taxa 2012-05-17 A 2011-08-31 B 2012-09-06 C I tried using: t.first <- species[unique(species$Taxa),] and it gave me the correct number of rows but there were Taxa repeated. If I just use unique(species$Taxa) it appears to give me the right answer, but then I don't know the date when it first occurred. Thanks for any help.