Subcutaneously implanted passive transponder tags have proved to be a reliable way of identifying individual penguins by both manual and automated means. We have found the survival of tagged Ad61ie penguins (Pygoscelis adeliae) over five seasons to be consistently higher than that of birds carrying flipper bands, although not statistically significant on an annual basis. There are, however, some problems associated with the use of implanted devices. The transponder removed from one bird had developed a slimy biofilm harbouring potentially pathogenic organisms incorporated at the time of implantation. If such contamination is common it is possible that the long-term survivorship of tagged groups of birds may actually be lower than that of untagged populations. Migration of tags away from the injection site may also compromise survival in some individuals. These risk factors could limit the use of implanted identification .devices in long-lived or endangered species. Introduction of bacteria can be minimised (but perhaps not eliminated) by careful injection techniques and cleansing of instruments and skin with iodine or alcohol. The problem of tag migration is yet to be quantified and may be more difficult to overcome. The use of implanted transponders in demographic studies is recommended as an alternative to flipper banding, but users should be aware that these identification markers may not be entirely problem-free.