Researchers tested 80 pools in the Majorcan resorts of Alcudia and Puerto de Pollensa and Sidari in Corfu.
Two of the pools were found to be contaminated with poo. Twenty of them were a health hazard and a further 29 were declared a possible health hazard.
Campaigners are now calling for stricter rules on keeping pools clean.
Some pool owners weren't putting any disinfectant in the water.
Others put in so much chlorine that it bleached people's swimming costumes and hurt their eyes and skin.
Just two of the pools were perfectly clean.
The people who carried out the survey described their findings as "stomach-churning".