Statistics Canada says manufacturing sales edged down 0.1% in April to $50.9 billion, the first decrease in four months.
The agency says declines in the petroleum and coal product, aerospace product and parts, and machinery industries were largely offset by smaller gains in most other industries.
Economists had expected a gain of 0.4%, according to Thomson Reuters.
In April, 14 of 21 industries representing about 60% of manufacturing posted gains, despite the slight decrease in total sales.
Sales in the petroleum and coal product industry decreased 5% in April to $6.9 billion, due to partial shutdowns at several refineries for maintenance and retooling. In contrast, sales in the motor vehicle industry rose 3.3% to $4.4 billion.
Sales tailed off in five provinces in April, with Quebec posting the largest decrease.