Police are linking two rapes in which victims were picked up by a taxi driver or someone claiming to be one.
Two women in their 20s were raped after nights out when they were picked up in Newbury, Berkshire, Thames Valley Police said.
A 40-year-old man from Newbury was arrested on suspicion of rape in connection with the attacks and has been released while inquiries continue.
Police described the attacker as "Asian" and "slim".
The first woman was attacked after she left the Vault nightclub in Kennet Shopping Centre on 19 January this year with a friend, but ended up alone, detectives said.
She was picked up in Mayors Lane at about 01:30 GMT on 20 January.
'Use licensed taxis'
Police said the woman was raped in the vehicle at an unknown location.
A 29-year-old woman was raped on 24 February after leaving the Coopers Arms in Bartholomew Street.
Officers said she got into a taxi near the roundabout at Inches Yard at about 04:20 GMT and was driven to a property at an unknown location where she was raped.
Police described the attacker as Asian, about 5ft 8in tall, in his 30s and slim.
Det Ch Insp Matt Stone said it was possible the attacker had "confided in someone about what they have done" and urged anyone with information to come forward.
Officers urged people to use licensed taxis and to book in advance or ask proof that a vehicle or driver is licensed.