An effective tagging technique represents a challenge for long-term studies in sea cucumbers. These studies require accurate identification, high tag retention and no adverse effects on vital rates or performance of the tagged individuals. We tested the retention rate of one external and one internal tag in the sea cucumber Holothuria mexicana. The passive integrated transponder (PIT) tag was inserted into the coelomic cavity and the T-bar (external tag) was attached in the upper surface of the body wall. Retention rate was lower, for both kinds of invasive tags, than required in long-term studies. Stress elicited by the tagging procedure caused evisceration of 40% of PIT-tagged individuals versus a 57% of T-bars during or no more than five minutes after tagging. No differences in evisceration between both tags were detected; nor any correlation between evisceration and length. To conclude, the tagging procedure harmed the animals and both PIT-tags and T-bars showed similar low retention rates after ten weeks. Both marks were not effective for long-term studies in Holothuria mexicana. Development of a technique to tag adult red king crab Paralithodes camtschaticus (Tilesius, 1815), with passive integrated transponder tags. Journal of Shellfish Research 11, 91-94. Duggan R.E. and Miller R.J. (2001) External and internal tags for the green sea urchin. Journal of Experimental Marine Biology and Ecology 258, 115-122. Ebert T.A. (2007) Growth and survival of postsettlement sea urchins. In Lawrence J.M. (ed.) Edible sea urchins: biology and ecology. 2nd edition. Oxford: Elsevier, pp. 95-134. Gibbons J.W. and Andrews K.M. (2004) PIT tagging: simple technology at its best. BioScience 54, 447-454. Goldman M. (2007) Migratory behavior of adult female blue crabs, Callinectes sapidus, in Pamlico Sound: implications for assessment and protection of the spawning stock.