This paper investigates disciplinary differences in how researchers use the microblogging site Twitter. Tweets from researchers in five disciplines (astrophysics, biochemistry, digital humanities, economics, and history of science) were collected and analyzed both statistically and qualitatively. The results suggest that researchers tend to share more links and retweet more than the average Twitter users in earlier research. The results also suggest that there are clear disciplinary differences in how researchers use Twitter. Biochemists retweet substantially more than researchers in the other disciplines. Researchers in digital humanities use Twitter more for conversations, while researchers in economics share more links than other researchers. The results also suggest that researchers in biochemistry, astrophysics and digital humanities are using Twitter for scholarly communication, while scientific use of Twitter in economics and history of science is marginal.