- Idealism
- identify
- Idiogram
- ileocaecal
- ileocolic
- ileum
- imbricate
- immature
- imprint
- inbreeding
- incisiform
- incisor
- inferior
- infraorbital foramen
- Ingroup
- inguinal
- insectivorous
- integument
- interbreed
- interdigital
- intergroup
- interparietale
- interstitial
- intestine
- intragroup
- intraspecific
- invertebrate
- iris
- Isalo
- Isalo massif
- Island arc
- Isolating mechanism
- Isometry
- Isozyme
- IUCN
- IUCN Categories of Threat
- IUCN Red List of Threatened Animals