Smouldering timbers at a former school in Llandysul caused a small fire to break out on Thursday.
A large fire broke out at the derelict building, which was being used as a £2 million cannabis farm up until earlier this month, on 17 July, which took crews from across west Wales more than six hours to extinguish.
Smouldering timbers inside the building reignited on Wednesday evening, causing a small fire to break out.
A spokesperson for Mid and West Wales Fire Service said: "At 8.53pm on Thursday, 25 July, Mid and West Wales Fire and Rescue Service crews from Llandysul and Newcastle Emlyn Fire Stations were called to an incident on Marble Terrace in Llandysul.
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"Crews responded to a small fire on the ground floor of a derelict building, formerly used as a primary school.
"The fire was caused by smouldering timbers from a previous fire at the building on Wednesday, 17 July.
"Crews utilised small gear to turn over the burning material and one hose reel jet to dampen it down.
"Crews left the scene at 9.50pm."
Dyfed-Powys Police is investigating the cause of the fire on 17 July, which it is treating as ‘unexplained’.