Size

• Female mollies are larger than their male counterparts.
  
  

Shape

• Male mollies have a slender, streamlined body. The female fish have a more rounded shape.
  
  

Fins

• Molly males have what is called a gonopodium. This is a modified anal fin that is used to help inseminate a female.
  
  

Fin Shape

• The female anal fin is more fanned out. The male's fin, the gonopodium, is perpendicular to his body.
  
  

Coloring

• When trying to gain the attention of females, males have more brilliant coloring. A female is a duller color, and, when pregnant, displays a dark spot on her belly.