Arnold, part of Havas Advertising, had prided itself on not cutting staff, but has been forced to reduce costs as part of the ad slowdown.
The cuts came across all levels and all departments at the network's Boston headquarters, said a spokeswoman, who cited the economy as the motivating factor.
The 20 cut this week represent about 3% of staff.
In the summer, Arnold closed its Cipriani Kremer Design unit, which housed about 15 people, and laid off about 30 people -- or 3% of staff -- across four offices. The San Francisco office also suffered layoffs because of the downturn in the tech sector.