As an adjective, obligate means "by necessity" (antonym facultative) and is used mainly in biology in phrases such as:
- Obligate aerobe, an organism that cannot survive without oxygen
- Obligate anaerobe, an organism that cannot survive in the presence of oxygen
- Obligate air-breather, a term used in fish physiology to describe those that respire entirely from the atmosphere
- Obligate biped, Bipedalism designed to walk on two legs
- Obligate carnivore, an organism dependent for survival on a diet of animal flesh.
- Obligate chimerism, a kind of organism with two distinct sets of DNA, always
- Obligate hibernation, a state of inactivity in which some organisms survive conditions of insufficiently available resources.
- Obligate intracellular parasite, a parasitic microorganism that cannot reproduce without entering a suitable host cell
- Obligate parasite, a parasite that cannot reproduce without exploiting a suitable host
- Obligate photoperiodic plant, a plant that requires sufficiently long or short nights before it initiates flowering, germination or similarly functions
- Obligate symbionts, organisms that can only live together in a symbiosis