Impax Asset Management Groups said assets under management totalled £8.2bn at the end of December - an increase of 13% since the end of October.
Impax said the figure did not include the assets managed by Pax World Management LLC, the acquisition of which the company announced on 18 Sep and which remained subject to customary closing conditions.
Chief executive Ian Simm said: 'I am pleased to report on another outstanding quarter.
'Investor interest in the transition to a more sustainable economy continues to gather momentum and it is most encouraging to see further strong inflows across our fund range from clients in Europe and North America.
'Our pipeline remains promising for both our listed and private equity strategies.'
At 8:23am: (LON:IPX) Impax Asset Management Plc share price was +9.5p at 181.25p