It flows up west of Hunsberget (232 m above sea level) in Ljungby and flows northwest towards lake Femmen (148 m above sea level) in Hylte Municipality.

From Femmen, the journey goes steadily southwest, past Kullhult, Bygget and Fröböke.

Then Fylleån turns first into Gyltigesjön and Töddesjön and then into the long narrow lake Simlången (65 m above sea level) in Simlångsdalen.

Lake Simlången turns into Brearedssjön from which Fylleån flows out and bends more and more to the west and finally flows intolerant Laholmsbukten at Fyllinge a few kilometers south of Halmstad and the Nissan mouth.

Furthermore, in Halmstad municipality there are the nature reserves Gårdshult and Danska adjacent to the river Ässman (or Assman), which flows into Brearedssjön, and thus forms a tributary to Fylleån River.