A surface treating appliance comprising: a first cyclonic separating unit comprising: at least one first cyclone; a perforated annular shroud having a lower wall and a side wall comprising an array of apertures providing a fluid outlet from the first cyclonic separating unit; and a first dust collector extending beneath the lower wall of the shroud for receiving dust from the first cyclonic separating unit; a second cyclonic separating unit located downstream from the first cyclonic separating unit, and comprising: a plurality of second cyclones arranged in parallel, each second cyclone having an upper section having a fluid inlet and fluid outlet, and a lower section having a dust outlet; and a second dust collector for receiving dust from the second cyclonic separating unit, the second dust collector being located beneath the dust outlets of the second cyclones and having an upper extremity lying 10 mm beneath the lowest extremity of the plurality of second cyclones; and a third cyclonic separating unit located downstream from the second cyclonic separating unit, and comprising: a plurality of third cyclones arranged in parallel, each third cyclone having an upper section having a fluid inlet and fluid outlet, and a lower section having a dust outlet; and a third dust collector for receiving dust from the third cyclonic separating unit, the third dust collector being located beneath the dust outlets of the third cyclones and having an upper extremity lying 10 mm beneath the lowest extremity of the plurality of third cyclones; wherein the volume of the second dust collector is greater than the volume of each of the first and third dust collectors. a first cyclonic separating unit comprising: at least one first cyclone; a perforated annular shroud having a lower wall and a side wall comprising an array of apertures providing a fluid outlet from the first cyclonic separating unit; and a first dust collector extending beneath the lower wall of the shroud for receiving dust from the first cyclonic separating unit; at least one first cyclone; a perforated annular shroud having a lower wall and a side wall comprising an array of apertures providing a fluid outlet from the first cyclonic separating unit; and a first dust collector extending beneath the lower wall of the shroud for receiving dust from the first cyclonic separating unit; a second cyclonic separating unit located downstream from the first cyclonic separating unit, and comprising: a plurality of second cyclones arranged in parallel, each second cyclone having an upper section having a fluid inlet and fluid outlet, and a lower section having a dust outlet; and a second dust collector for receiving dust from the second cyclonic separating unit, the second dust collector being located beneath the dust outlets of the second cyclones and having an upper extremity lying 10 mm beneath the lowest extremity of the plurality of second cyclones; and a plurality of second cyclones arranged in parallel, each second cyclone having an upper section having a fluid inlet and fluid outlet, and a lower section having a dust outlet; and a second dust collector for receiving dust from the second cyclonic separating unit, the second dust collector being located beneath the dust outlets of the second cyclones and having an upper extremity lying 10 mm beneath the lowest extremity of the plurality of second cyclones; and a third cyclonic separating unit located downstream from the second cyclonic separating unit, and comprising: a plurality of third cyclones arranged in parallel, each third cyclone having an upper section having a fluid inlet and fluid outlet, and a lower section having a dust outlet; and a third dust collector for receiving dust from the third cyclonic separating unit, the third dust collector being located beneath the dust outlets of the third cyclones and having an upper extremity lying 10 mm beneath the lowest extremity of the plurality of third cyclones; a plurality of third cyclones arranged in parallel, each third cyclone having an upper section having a fluid inlet and fluid outlet, and a lower section having a dust outlet; and a third dust collector for receiving dust from the third cyclonic separating unit, the third dust collector being located beneath the dust outlets of the third cyclones and having an upper extremity lying 10 mm beneath the lowest extremity of the plurality of third cyclones; wherein the volume of the second dust collector is greater than the volume of each of the first and third dust collectors.