Try this, works on 1.9.5 dt[,.(list(V1)),.(V2)] V2 V1 1: a 1,4 2: b 2,3 -- View this message in context: http://r.789695.n4.nabble.com/data-table-Aggregate-as-vector-tp4709068p4710244.html Sent from the datatable-help mailing list archive at Nabble.com.