Thames Water has been issued with a £250,000 fine for polluting a brook in Newbury.
It has also been ordered to pay costs of nearly £7,000 by Reading Court Crown after untreated sewage flowed into Chase Brook.
The Environment Agency brought the case after sewage “severely impacted” wildlife in the brook in September 2012.
Thames Water pleaded guilty and said it has made changes at the site since the incident.
Matthew Rice, Environment Agency Environment Officer said: “Water quality testing revealed that there were high levels of both ammonia and e-coli in the pond and Brook. This combined with low oxygen levels meant that recovery from the incident was a long process.
“Ammonia is toxic to aquatic organisms and low dissolved oxygen levels starve creatures of oxygen which is of course essential for life”.