Lifeguards in Merseyside have reported a big increase in the number of stinging jellyfish off the area's coastline.
Jellyfish have been attracted to coastal areas by the warm weather
Hot weather and warmer seas have attracted the creatures into the Mersey estuary.
Experts say thousands have been spotted already, making 2003 the worst year in the past decade.
The main culprits are the Lion's Mane and Compass varieties, both of which deliver a nasty sting.
Twelve stings were reported in Wirral over the weekend and lifeguards have urged bathers to take extra care.
Chris Ince, of Seacombe Aquarium, said: "In 12 years I have never seen a year like this for these two types of stinging jellyfish that we are getting.
"Normally they're the little harmless Moon jellyfish, but at the moment we are finding a lot of Lion's Mane jellyfish, and they bring a really nasty sting."
Experts say those stung should pour vinegar on the affected area, which kills off the stinging cells.
For really bad cases, medical attention is advised.