A number of dead ducks have been found floating head down on a waterway in Lincolnshire.
About a dozen dead ducks have been spotted in the past week in the North Forty Foot Drain, in Boston.
Lincolnshire Police confirmed reports from local dog walkers that someone had previously been spotted pouring "a liquid" into the water.
Defra said it has taken samples from the water, which are currently being examined.
It follows a recent outbreak of avian botulism at a beauty spot in Doncaster, which was blamed for the deaths of more than 30 swans.
RSPCA inspector John Gibson said the illness was down to the recent hot weather causing low water levels that led to rotting vegetation increasing the level of toxic algae.
In a statement, Defra confirmed reports that dead wild birds had been sighted in the drain.
However, it said it was too early to say what was responsible and it would announce its findings in due course.