Occupational therapist is the hardest occupation to fill in the US for the 12 months ended June 2015, according to The American Staffing Association’s skills gap index, a quarterly index that tracks hard-to-fill occupations in the US.

During the past quarter, microbiologists and surgeons moved into the top 10 list of hardest-to-fill positions, displacing merchandise displayers and window trimmers, and physician assistants.

The index identified 172 occupations as hard to fill. The top 10 are:

1. Occupational therapists
2. Heavy and tractor-trailer truck drivers
3. Speech-language pathologists
4. Photographic process workers and processing machine operators
5. Physical therapists
6. Occupational therapy assistants
7. Family and general practitioners
8. Microbiologists
9. Surgeons
10. Nurse practitioners

“Demand for staffing and recruiting services continues to grow as labor markets tighten and companies place a higher strategic focus on talent as a key business differentiator,” said ASA President and CEO Richard Wahlquist. “Partnering with staffing and recruiting firms to develop and implement new and more effective workforce solutions helps companies more successfully source qualified talent and narrow their skills gaps.”

ASA developed the index utilizing CareerBuilder’s hiring indicator, which measures the level of difficulty — on a scale of one to 100 — to recruit for a specific occupation based on demand, supply of active candidates and total population working in it. The ASA skills gap index concentrates on hiring indicator scores of 50 or less for hard-to-fill occupations, with a demand of 2,000 jobs or more.