A case of monstra duplicia is recorded in a laboratory population of Tetranychus urticae. The population had not been previously exposed to teratogenic agents. Developmental abnormality was found in the protonymph stase. The setae, stylets and leg articles are approximately 10-20% shorter than in normal protonymphs. Ventral and dorsal chaetotaxy and all anal structures have also been modified, as the consequence of the duplicated last 6 segments, i.e. the hysterosoma. Legs IV have been duplicated, and III partly so, by missing the one of them and deforming the other, inner one.