A study of the breeding cycle and population structure of Angeliera phreaticola (Isopoda, Asellota, Microparasellidae) has been carried out in the western Mediterranean. The species shows a seasonal reproductive cycle. The breeding season occurs from mid-April to the end of September. Release of juveniles is limited to the period from June to the end of September. Fourty to seventy days are necessary for the embryological development which is very long, eighty days for the post-marsupial one. It is suggested that in spring the increasing temperature of the interstitial waters accelerates the maturity of the ovocyte and post-embryonic development, and causes an advance of the breeding season. Each summerborn generation reproduces next year and yields reproducing animals two years after. Each female produces two broods (rarely three) per reproduction season and can get three to six descendants at most. Sex ratio of males to females is expressed as a function of the season and the size; males outnumber females. A. phreaticola has a maximum life span of about two years and two or three months.