The Minnesota Pollution Control Agency is warning swimmers and pet owners to keep a lookout for blue-green algae, KSTP-TV reports.
The station says the toxic substance has already been spotted on lakes in the southern and western part of the state, and is now spreading to the Twin Cities area.
The 90-degree-plus temperatures of late have spurred growing conditions for blue-green algae, which can prove deadly to pets and make humans very sick.
The MPCA says humans and animals may develop skin irritation or upper respiratory problems if exposed to the toxins. In some cases, dogs and other animals have died drinking lake water containing blue-green algae.
An official from the MPCA says blue-green algae looks like oil paint on the surface of water, and can be blue, green or gray as the substance decomposes.
See KSTP's report below.