We use a French firm-level data set containing 13,000 firms over the period 1993-2004 to analyze the relationship between credit constraints and firms' R&D behavior over the business cycle. Our main results can be summarized as follows: (i) the share of R&D investment over total investment is countercyclical without credit constraints, but it becomes less countercyclical as firms face tighter credit constraints; (ii) this result is magnified for firms in sectors that depend more heavily upon external finance, or that are characterized by a low degree of asset tangibility ; (iii) in more credit constrained firms, R&D investment share plummets during recessions but does not increase proportionally during upturns; (iv) average R&D investment and productivity growth are more negatively correlated with sales volatility in more credit constrained firms.