Ioanna T. Morfessis, whose successful career as head of Montgomery County's economic development office was marred by a costly county investigation into allegations that she illegally promoted two employes, has resigned her position to take a similar post in Phoenix.

A Greek immigrant's daughter who helped make her native Montgomery County a national center for high-technology and biomedical firms, Morfessis, 34, will be the first executive director of the newly formed Phoenix Economic Growth Corp., a joint venture between government and private business there.

Morfessis announced her resignation in a letter this week to Montgomery County Executive Charles W. Gilchrist, saying she was leaving her job in the Maryland suburb "with a heavy heart."

Morfessis, whose county job paid $58,000 after a recent performance bonus from Gilchrist, will earn $75,000 in her new post plus a benefits package worth about $15,000.

William H. Mallender, a prominent Phoenix businessman who chairs the corporation, said Morfessis was selected from a field of more than 100 persons who sought the nationally advertised job.

Morfessis was selected, Mallender said, partly because of Montgomery's nationwide reputation for orderly growth and its ability to attract so-called "clean" industry.

In 1983, Morfessis became the target of a county council-backed investigation into allegations that she had decided to hire two people who had worked in her office as employes under the U.S. Comprehensive Employment and Training Act (CETA) before they formally applied for jobs. Under Montgomery's merit personnel system, department heads are barred from "wiring" jobs to ensure the hiring of favored applicants.

At the same time, U.S. Labor Department investigators alleged that under Morfessis' supervision, the county's $4.6 million local CETA program had improperly spent roughly $100,000 on wages, contracts and office rent.

The county investigation, which cost more than $150,000 and was conducted by the government's merit system protection board, ended last September. The key witness in the case -- one of Morfessis' former employes -- quoted her as instructing the employe to rig jobs.

Morfessis repeatedly denounced the central charge as untrue. Gilchrist dimissed the investigation as "abusive" and no formal charges were brought in the case.

The federal audit of Morfessis' office also ended last year. The county, saying the improprieties alleged were not out of line for such a large program, agreed to reimburse the federal government for part of the $100,000.