| Acanthodii | EUKARYA>UNIKONTA>OPISTHOKONTA>ANIMALIA>BILATERIA>DEUTEROSTOMATA>VERTEBRATA>ACANTHODII |
| Acoelomorpha | EUKARYA>UNIKONTA>OPISTHOKONTA>ANIMALIA>ACOELOMORPHA |
| Actinobacteria | EUBACTERIA>FIRMICUTAE>ACTINOBACTERIA |
| Actinophryida | EUKARYA>CHROMALVEOLATA>HETEROKONTAE>ACTINOPHRYIDA |
| Alphaproteobacteria | EUBACTERIA>PROTEOBACTERIAE>ALPHAPROTEOBACTERIA |
| Alveolatae | EUKARYA>CHROMALVEOLATA>ALVEOLATAE |
| Amoebozoae | EUKARYA>UNIKONTA>AMOEBOZOAE |
| Anapsida | EUKARYA>UNIKONTA>OPISTHOKONTA>ANIMALIA>BILATERIA>DEUTEROSTOMATA>VERTEBRATA>ANASPIDA |
| Anaspidomorphi | EUKARYA>UNIKONTA>OPISTHOKONTA>ANIMALIA>BILATERIA>DEUTEROSTOMATA>VERTEBRATA>ANASPIDOMORPHI |
| Ancylistaceae | see Entomophthoromycota |
| Andraeopsida | see Bryophyta |
| Angiospermophyta | EUKARYA>ARCHAEPLASTIDA>VIRIDIPLANTAE>STREPTOBIONTA>EMBRYOPHYTA>TRACHEOPHYTA>SPERMOPHYTA>ANGIOSPERMOPHYTA |
| Animalia | EUKARYA>UNIKONTA>OPISTHOKONTA>ANIMALIA |
| Anita Grade | see Nymphaeopsida |
| Annelida | EUKARYA>UNIKONTA>OPISTHOKONTA>ANIMALIA>METAZOA>BILATERIA>PROTOSTOMATA>SPIRALIA>TROCHOZOA>EUTROCHOZOA>ANNELIDA |
| Anoxybacteria | EUBACTERIA>FIRMICUTAE>ANOXYBACTERIA |
| Anthocerotophyta | EUKARYA>ARCHAEPLASTIDA>VIRIDIPLANTAE>STREPTOBIONTA>EMBRYOPHYTA>ANTHOCEROTOPHYTA |
| Anthocerotopsida | see Anthocerotophyta |
| Aphragmabacteria | EUBACTERIA>FIRMICUTAE>APHRAGMOBACTERIA |
| Apicomplexa | EUKARYA>CHROMALVEOLATA>ALVEOLATAE>APICOMPLEXA |
| Apusozoa | EUKARYA>UNIKONTA>AMOEBOZOA>AMOEBOZOAE>?APUSOZOA |
| Arachnomorpha | see Arthropoda |
| Archaea | ARCHAEA |
| Archaeplastida | EUKARYA>ARCHAEPLASTIDA |
| Archosauromorpha | EUKARYA>UNIKONTA>OPISTHOKONTA>ANIMALIA>BILATERIA>DEUTEROSTOMATA>VERTEBRATA>GNATHOSTOMATA>TETRAPODA>AMNIOTA>DIAPSIDA>ARCHOSAUROMORPHA |
| Arthropoda | EUKARYA>UNIKONTA>OPISTHOKONTA>ANIMALIA>BILATERIA>PROTOSTOMATA>ECDYSOZOA>PANARTHROPODA>ARTHROPODA |
| Ascomycota | EUKARYA>OPISTHOKONTA>UNIKONTA>FUNGI>DIKARYA>ASCOMYCOTA |
| Asteropsida | EUKARYA>ARCHAEPLASTIDA>VIRIDIPLANTAE>STREPTOBIONTA>EMBRYOPHYTA>TRACHEOPHYTA>SPERMOPHYTA>ANGIOSPERMOPHYTA>ASTEROPSIDA |