Fire crews are battling a blaze in an industrial unit in Basin Road South, at the eastern end of Shoreham Harbour.
The building of Brighton and Newhaven Fish Sales, next to Hove Lagoon, is understood to be on fire.
An East Sussex Fire and Rescue spokesman said four crews are fighting the fire in the first floor and in the loft space.
He said crews were called to the blaze at 7.29 this evening by someone driving past the building who spotted smoke.
At 8.30pm The Argus chief photographer Simon Dack said: "About eight fire engines are here. It appears the fire has broken out in the offices rather than the part of the building where the fish is kept.
"Windows on the first and ground floor are blackened, but it appears to all have been brought under control."
On its website the company describes itself as a "wholesale and retail fish outlet".
It continues: "We sell high quality, day-caught local fish and shellfish, from our fleet of inshore fishing boats, direct to both wholesale customers (throughout the UK and Europe) and to the public through our new Fish shop that is located right on the quay side.
"We pride ourselves on high standards of product and service and are able to give all our customers a guarantee of provenance with all fish purchased from us.
"This is unique within our industry as we are the only UK fish market that owns fish quotas and are therefore able to stand behind this statement."