Around 50 frogs sparked an unlikely rescue operation when they swamped an underground shopping car park.
An RSPCA inspector was called out to Aldi in Finchley, north London, after reports of the amphibian invasion at 9.30pm on Saturday.
They had apparently been dumped in a plastic bag full of soil and algae, but more than 20 had escaped and were hopping around, the wildlife charity said.
Forty-eight of the frogs were rescued successfully, but two died after being run over.
Inspector Lauren Evans said: "When I arrived there were all these frogs jumping around the dark car park - and I had to run around scooping them all up with my hands and a bag before placing them into a secure tank.
"Some were still in the plastic bag, which had clearly been tied up intentionally. They would not have been able to escape if there had not been the hole, but equally they would not have been able to breathe without it.
"It was clearly a completely hazardous environment for them to have been left in - with cars driving around unable to see them and nowhere for them to escape to. Two had sadly been run over and killed, but I got there just in time for the others.
"I have no idea where these frogs might have come from or why someone would put them in a plastic bag and then leave them in a car park like this - but none of them would have survived if they had been left that way."
They were released back into the wild after being given a medical check at Putney Animal Hospital.