We report gene expression patterns for the flatworm Macrostomum lignano in four different social envirionments expected to influence allocation to the male and female sex functions Overall design: Four replicate pools of worms worm raised in each of four different treatment groups: 'isolated' worms were kept alone for the duration of the experiment 'joined' were kept alone for the duration of the experiment, except that for the final 24h always two worms were paired together (aimed at studying short-term responses to mating) 'paired', in which two worms were kept together for the duration of the experiment and 'octet' , in which worms were kept together in groups of eight for the duration of the experiment. After 64 days in these groups, RNA was extracted and pooled to form the 16 independent cDNA libraries for sequencing on the Illumina HiSeq 2000 platform (single-end, 100bp reads).