How to tell if a guppy is male or female

Why it matters

Guppies are livebearers — they give birth to free-swimming fry. If you keep males and females together, you will get babies. Knowing how to sex guppies helps you control breeding, avoid overcrowding, and plan your stocking. It also matters for compatibility: too many males chasing too few females causes stress.

The anal fin (gonopodium)

The most reliable way to sex guppies is the anal fin — the small fin on the underside, just behind the belly.

• Male: The anal fin is long, narrow, and pointed. It looks like a rod or stick. This is the gonopodium, the organ used to fertilise females.
• Female: The anal fin is triangular or fan-shaped. It is shorter and broader than the male's.

In young guppies, the gonopodium develops as they mature. Juveniles can be harder to sex; wait until they are a few weeks old.

The gravid spot

Females have a dark patch near the anal vent, behind the belly. This is the gravid spot — it darkens and enlarges when the female is pregnant. Males do not have a gravid spot. On light-coloured or albino females, the spot can be subtle but is usually visible.

Body size and shape

Females are larger and rounder, especially when pregnant. Males are slimmer and smaller. A mature female can be almost twice the length of a male. Her belly may bulge noticeably when carrying fry.

Colour and fins

Male guppies are usually more colourful: bright tails, spotted bodies, and vivid patterns. Females are plainer — grey, silver, or lightly coloured. Male tails and dorsal fins are often larger and more elaborate. These traits vary by strain; some fancy females have more colour, but the anal fin and gravid spot remain the best identifiers.

Quick reference

• Male: Pointed anal fin (gonopodium), smaller, brighter, no gravid spot.
• Female: Triangular anal fin, larger, rounder, gravid spot near vent.

What to do with the information

If you want to avoid fry, keep only males or only females. All-male tanks work well for colour without breeding. If you want fry, keep a ratio of at least two females per male to spread harassment. See our guppy care guide and breeding for more. Use App-aquatic's stocking calculator to plan your tank before adding fish.