Thousands of litres of diesel was found to have spilt into the river from tanks at the First Western National Buses depot last November, and the firm had to bring in a specialist firm to clean up the pollution.
Speaking after magistrates fined FWNB £2,500 with £797 costs for leaking diesel into the River Taw in Barnstaple, a spokesman for the Environment Agency said: 'Everyone with responsibility for storing fuel oils must take that responsibility seriously and regularly check tanks to ensure they are not leaking.
'Last year a sixth of all pollution incidents involved oil and as shown by this case it can be extremely complicated and costly to clean up.'
Strict new regulations have been put in place for any company storing more than 200 litres of oil above ground at industrial and commercial sites. A free leaflet on the regulations is available from the Environment Agency by calling 0845 9333111.