| | 1. Leaf petioled, blade ovate to reniform; flower bisexual, emergent |
| OTTELIA (no key) |
| | 1' Leaf sessile, blade thread-like to oblong; flowers unisexual or bisexual, at water surface or submersed — staminate flowers sometimes deciduous, free-floating |
| |
| | 2. Flower sessile, 1–few per axil; perianth 0; fruit achene with net-like surface |
| NAJAS |
| | 2' Flower with perianth tube peduncle-like, sepals green, petals colored or white; fruit berry-like and dehiscing irregularly, surface not net-like |
| |
| | 3. Leaves 2–4 cm, 2–5 mm wide, generally 3–6 per node; staminate flowers 2–3, petals 8–10 mm |
| EGERIA (no key) |
| | 3' Leaves < 1.5(2) cm, < 3 mm wide, opposite or 2–3 per node; staminate flower 1, petals generally < 5 mm |
| |
| | 4. Leaf margin minutely and finely serrate, midrib on lower surface ± smooth |
| ELODEA |
| | 4' Leaf margin clearly and sharply serrate, midrib on lower surface ± rough |
| HYDRILLA (no key)
|