The Surdo is a large bass drum used in many kinds of Brazilian music, most notably samba.

Surdo sizes normally vary between 16" and 22" diameter. In Rio de Janeiro, surdos are generally 60cm deep. Surdos used in the northeast of Brazil are commonly more shallow (50cm deep). Surdos may have shells of wood, galvanized steel, or aluminum. Heads may be goatskin or plastic. Rio baterias commonly use surdos that have skin heads (for rich tone) with aluminum shells (for lightness). Surdos are worn from a waist belt or shoulder strap, oriented with the heads roughly horizontal. The bottom head is not played.

A typical Carnival samba bateria in Rio de Janeiro has three surdo parts distinguished by tunings (and, by extension, sizes of drum). Together these three parts create a distinctive pattern which propels and drives the samba.

The surdos on which the primeira ("first") or marcação ("marker") part is played are the largest and deepest-pitched drums in the bateria. The primeira part provides the pulse or rhythmic reference for the entire bateria. It sounds the "2" of the basic "1, 2" rhythm of samba and may also sound pick-up notes to start the music.

The primeira is answered by a slightly smaller and higher-pitched surdo playing a part known as segunda ("second"), resposta ("response"), or respondor ("that which responds"). It is generally played on drums 20" or 22" in diameter. It sounds the "1" of the basic "1, 2" rhythm of samba.

The third surdo part, called terceira ("third") or cortador ("cutter" in English), is played on the smallest and highest-pitched surdos, generally between 14" and 18" in diameter. The terceira part consists of more complex patterns that provide fills and syncopations, producing an effect of "cutting" across the basic pulse created by other two surdo parts. Terceira patterns are important drivers of music's "swing" — the feel of the bateria. Terceira players are the only surdo players with room for limited improvisation.