Inflows into its funds were a record £1.8bn in the half-year to March 31, 2020, Impax said, but assets under management were affected by coronavirus (COVID-19) volatility and dropped by 4% to £14.4bn.
In April, net inflows were almost £300mln, it added, and at the end of the month assets under management had rebounded to £15.8bn.
In the interim results statement, Ian Simm, Impax AM's chief executive said: "Over the past six months, Impax's financial performance has been strong, with high levels of net inflows.
“Despite market volatility arising from the COVID-19 crisis, investor interest in our funds has remained robust as asset owners look for attractive investment returns, resilient portfolios and the prospect of positive environmental and social impact."
Simm added that the thematic (environmental markets) and Global Opportunities funds had seen the bulk of the inflows.
Revenues for the half-year were £41.2mln, up from £33.8mln at the same stage in its previous financial year, while profits before tax dropped to £8mln from £9.3mln.
The interim dividend went up to 1.8p, from 1.5p, and Impax said despite the impact of COVID-19 it is in good financial health so the board decided to proceed with a payment at the lower end of its previously indicated range.
Simm noted that the group is expecting a difficult period for markets ahead but that this will favour its themes of sustainable development.
He said: “Current market levels appear to be pricing in a global recession for at least the next year. In the short-term it is inevitable that we will see widespread dividend cuts and cash calls from many companies.
"As the global recession triggered by COVID-19 unfolds, there is mounting evidence that future consumer preferences and government regulation will align even more closely with the requirements of sustainable development."