The largest network ties together more than 350,000 accounts and further work suggests others may be even bigger.
UK researchers accidentally uncovered the lurking networks while probing Twitter to see how people use it.
Some of the accounts have been used to fake follower numbers, send spam and boost interest in trending topics.
On Twitter, bots are accounts that are run remotely by someone who automates the messages they send and activities they carry out. Some people pay to get bots to follow their account or to dilute chatter about controversial subjects.
"It is difficult to assess exactly how many Twitter users are bots," said graduate student Juan Echeverria, a computer scientist at UCL, who uncovered the massive networks.