More than 30 chickens found dead on an allotment may have been fed contaminated corn.
Blood on feathers and frothing indicated the birds had been poisoned, said RSPCA inspectors.
The animals were found at an allotment in Brithdir, near New Tredegar, Caerphilly, on 12 November.
The animal charity said the deaths were "highly suspicious and disturbing." The organisation called on anyone with information to contact them.
RSPCA inspector, David Milborrow, said: "This was a gruesome discovery - with dozens of chickens found dead, having been frothing at the mouth and with bloody combs, at this Brithdir allotment.
"We fear the chickens would have suffered immeasurably by any poisoning."
The birds belonged to two men who have allotments at the Tennyson Terrace site. One of the owners discovered the bodies.
"The feeling is that someone has been putting down contaminated corn," added RSPCA officials.
"We are appealing for information. We do not know why they did they did this."
"The corn that was contaminated has not yet been tested. My understanding is that the people who own the chickens do not put corn down.
"We think someone has come and put corn down with the intention of harming the chickens."