Southern Water has been fined £500,000 for polluting the watercourse in Kent with untreated sewage that killed local wildlife.
It has also agreed to pay costs of more than £19,000 after the 1.2 kilometre stretch of Swalecliffe Brook was affected.
Canterbury Crown Court was told the company did not respond quickly enough to the pollution in July 2013, which was caused by problems at a pumping station.
Nearly 250 fish, including endangered eels were found dead.
Alan Cansdale, Environment Manager at the Environment Agency said: “Southern Water has acknowledged it had sufficient warning and knowledge of the incident to minimise the impact on the local environment but failed to act swiftly and notify partners to help this happen.”