The magnetic properties of Zn1−xCoxS (x=0.025 and 0.05) thin films grown on α-quartz substrates at different temperatures (TS) of 200, 400 and 600 °C by means of pulsed laser deposition are presented. The films are crystallized with wurtzite structure. Optical absorption and transmission electron microscopy measurements indicate that Co ions are substituted to Zn on tetrahedral sites. Their magnetic response is composed of ferromagnetic and paramagnetic components of which respective strengths depend on TS and Co concentration. This behavior is interpreted as due to fluctuations in the magnetic ordering, depending on grain size and site location in grain boundaries or in crystal cores.