Largemouth vs Smallmouth Bass: How to Tell Them Apart

Largemouth and smallmouth bass are the two most popular gamefish in North America, and despite the names, size is not how you tell them apart. A small largemouth and a big smallmouth get confused constantly. Here are the reliable tells — the kind that hold up regardless of the fish's age, mood or the water it came from.

The jaw test (the one that always works)

Close the fish's mouth and look at the back corner of the upper jaw (the maxilla) relative to the eye:

• Largemouth: the jaw extends past the back edge of the eye. Big bucket mouth.
• Smallmouth: the jaw ends at or before the middle of the eye. Noticeably smaller mouth.

This is the single most reliable feature. If you only remember one thing, remember the jaw line and the eye.

The dorsal fin

Look at the spiny front part of the dorsal fin and the soft back part. On a largemouth, the two parts are nearly separated by a deep notch — it almost looks like two fins. On a smallmouth, the dorsal fin is connected by a shallow notch and reads as one continuous fin.

Colour and markings

• Largemouth: usually green, with a dark horizontal stripe (a series of blotches) running along the side.
• Smallmouth: usually brown to bronze (hence "bronzeback"), with vertical bars and often red-tinted eyes.

Use colour only as a tie-breaker — water clarity and habitat shift it a lot. A smallmouth from a stained river can look surprisingly green. Trust the jaw and the dorsal fin first. For more on why colour is the least reliable clue, see How to Identify a Fish From a Photo.

Behaviour and habitat

It is not identification, but it is a useful prior. Largemouth favour warmer, slower, weedier water — ponds, lake bays, vegetation. Smallmouth prefer cooler, clearer water with rock and current — rivers and the rocky parts of lakes. And anyone who has caught both will tell you the smallmouth fights pound-for-pound harder, with more jumps.

Quick reference

1. Jaw past the eye? Largemouth. Jaw to the eye? Smallmouth.
2. Dorsal fin deeply notched (almost two fins)? Largemouth. Shallow notch (one fin)? Smallmouth.
3. Horizontal stripe? Largemouth. Vertical bars and bronze body? Smallmouth.

Want it confirmed instantly? Photograph your catch in CastLog and the AI reads the jaw, fins and body profile for you, names the species, estimates the weight and length, and logs it to your journal — wherever you fish across the US and UK.