Alismatidae is a botanical name at the rank of subclass. Circumscription of the subclass will vary with the taxonomic system being used (there are many such systems); the only requirement being that it includes the family Alismataceae. It is a relatively new name: earlier systems, such as the Engler and Wettstein systems, used the name Helobiae for a comparable unit.

The Takhtajan system treats this as one of six subclasses within the class Liliopsida (=monocotyledons). It consists of:

• subclass Alismatidae

  superorder Alismatanae

  order Butomales

  order Hydrocharitales

  order Najadales

  order Alismatales

  order Aponogetonales

  order Juncaginales

  order Potamogetonales

  order Posidoniales

  order Cymodoceales

  order Zosterales

The Cronquist system treats this as one of four subclasses within the class Liliopsida (=monocotyledons). It consists of (1981):

• subclass Alismatidae

  order Alismatales

  order Hydrocharitales

  order Najadales

  order Triuridales

This subclass comprises less than five hundred species total: many of these are aquatic or semiaquatic plants (see Alismatidae info).

The APG II system does not use formal botanical names above the rank of order; it assigns most of the plants involved to the (expanded) order Alismatales, in the clade 'monocots', although the plants in Cronquist's order Triuridales are assigned to quite different placements.