Where the cost and uncertainty of R & D are high and conditions favor speedy imitation by competitors, concentration has a statistically significant and substantial positive impact on research intensity. But its impact is clearly negative when R & D cost and uncertainty are low, and where high barriers to imitation exist. A sample of 160 business unit observations for 1978 was drawn from the PIMS Data Base. All the sample business units' industries recently experienced major technological changes. Research intensity was measured by non-government-financed R & D expenditures as a percentage of sales.