Aims: To determine the occurrence and levels of Cryptosporidium parvum oocysts in wastewater and surface waters in north-eastern Spain. Methods and Results: Samples from five sewage treatment plants were taken monthly and quarterly during 2003. In addition, water was collected monthly from the River Llobregat (NE Spain) during the period from 2001 to 2003. All samples were analysed by filtration on cellulose acetate filters or through Envirocheck (TM) using EPA method 1623, followed by immunomagnetic separation and examination by laser scanning cytometry. All raw sewage, secondary effluent and river water samples tested were positive for Cryptosporidium oocysts. Of the tertiary sewage effluents tested, 71% were positive for Cryptosporidium oocysts. The proportion of viable oocysts varied according to the sample. Conclusions: Two clear maxima were observed during spring and autumn in raw sewage, showing a seasonal distribution and a correlation with the number of cryptosporidiosis cases and rainfall events. Significance and Impact of the Study: This study provides the first data on the occurrence of Cryptosporidium oocysts in natural waters in north-eastern Spain.